Chelsea owner Roman Abramovich says he is planning to sell the club.
In a statement on the Premier League club's website, businessman Abramovich said he had made "incredibly difficult decision" which "pains" him.
The Russian will not ask "for any loans to be repaid" and said proceeds of the sale would be donated to war victims.
Abramovich had said on Saturday he would give "stewardship and care" of Chelsea to its foundation trustees following Russia's invasion of Ukraine.
That led to speculation Abramovich - who has loaned the club more than £1.5bn - would put Chelsea up for sale, and billionaire Hansjorg Wyss told Swiss newspaper Blick on Wednesday that he had been offered the chance to buy the club.
Wyss said Abramovich wanted "to get rid of Chelsea quickly" after the threat of sanctions was raised in Parliament.
Abramovich, 55, is alleged to have strong ties to Russian President Vladimir Putin, which he has denied.
He said "all net proceeds from the sale" would be donated to the "victims of the war in Ukraine".
BBC Sport understands Abramovich has already received offers for Chelsea and that he values the club at as much as £3bn.
In his statement, Abramovich said: "I have always taken decisions with the club's best interest at heart.
"In the current situation, I have therefore taken the decision to sell the club, as I believe this is in the best interest of the club, the fans, the employees, as well as the club's sponsors and partners.
"The sale of the club will not be fast-tracked but will follow due process. I will not be asking for any loans to be repaid.
"I have instructed my team to set up a charitable foundation where all net proceeds from the sale will be donated. The foundation will be for the benefit of all victims of the war in Ukraine."
BBC Sport understands the trustees of Chelsea's charitable foundation, who had not yet agreed to take control of the club, will no longer be asked to. Abramovich was criticised for not referencing Russia's invasion of Ukraine in that original statement on Saturday.Chelsea's players were not told ahead of time about their owner's new statement, which was released less than a hour before the side's FA Cup fifth-round tie at Luton Town.
After falling behind early on, the Premier League team won 3-2 to reach the quarter-finals.
Abramovich bought Chelsea in 2003 for £140m, and in his statement said it had "never been about business nor money, but about pure passion for the game and club".
Since his purchase of the club, Chelsea have been transformed, setting the template for how much finance was needed to compete at the top end of the Premier League.
In total, Abramovich has loaned the club more than £1.5bn, which has helped to bring great success.
Under his ownership, the club have won every major trophy possible - including the Champions League twice, both the Premier League and FA Cup five times, the Europa League twice and the League Cup three times.
In August 2021, they won the Uefa Super Cup and in February won their first Club World Cup.
He has appointed 13 different managers and the club spent more than £2bn in the transfer market under his ownership.
Chelsea's women's team, who became affiliated to Chelsea FC in 2004, have thrived under Abramovich's ownership, winning the Women's Super League four times, the Women's FA Cup, three times and reaching the Champions League final last season.
Their high profile has seen world stars such as Australia's Sam Kerr and Danish forward Pernille Harder move to the club.
Other Russian billionaires have already been the subject of European Union sanctions where their assets have been frozen.
That has included Russian billionaire Alisher Usmanov, who has commercial links to Everton.
The United Kingdom government is yet to sanction Abramovich or Usmanov, but Labour MP Chris Bryant said in Parliament on Tuesday that Abramovich was "terrified of being sanctioned which is why he is going to sell his home tomorrow [Wednesday], and another flat as well".
On Wednesday, during Prime Minister's Questions, Labour leader Keir Starmer asked why Abramovich was not facing sanctions, with Boris Johnson replying it was "not appropriate to comment on any individual cases at this stage".
A spokesperson denied the Labour leader's claims and said Abramovich had not done anything sanctionable.
Wyss also said: "Abramovich is trying to sell all of his villas in England, he also wants to get rid of Chelsea quickly.
"I and three other people received an offer on Tuesday to buy Chelsea from Abramovich."
A spokesperson for Abramovich declined to comment on those claims.
Dan Roan, BBC sport editor
While this was not an entirely unexpected development after it became apparent during the last 24 hours that Roman Abramovich was looking to sell Chelsea, this dramatic announcement still feels like a seismic moment. Not only for the west London club, but for the wider English game too.
The Russian billionaire's arrival almost two decades ago helped transform the Premier League's competitive balance, profile and spending power, reinforcing its status as a truly global competition.
The oligarch's riches turned Chelsea into a major force in the game, and his ownership seemed to pave the way for subsequent takeovers by overseas billionaires, at clubs like Manchester City for instance.
But there was significant controversy too.
With Abramovich - one of Russia's richest people - almost never commenting in public, he also came to symbolise the detachment of some super-wealthy club owners. Many also found the ruthlessness with which he treated managers questionable.
There was always intrigue surrounding the motives for his ownership, and how he made his fortune. While he has always denied direct links with Putin, the invasion of Ukraine led to calls in parliament for the government to sanction wealthy Russians.
Abramovich vehemently denies that he had done anything to merit sanctions being imposed against him. But as the highest-profile oligarch in the country, and with one MP citing concerns about alleged links to corruption, his ownership was coming under mounting scrutiny.
And with the government facing mounting pressure to seize his assets, and Abramovich criticised over his failure to explicitly condemn Russia's invasion, it is little surprise that he has swiftly announced his decision to sell.
In recent years Abramovich has been an increasingly infrequent visitor to Stamford Bridge, withdrawing his application for a visa in 2018 after relations between the UK and Russia worsened.
But while fans had got used to not seeing Abramovich, the Russian's departure could mean the club is run very differently in future - and perhaps with much more financial constraint - by whoever now buys it.
That may help explain why some at Chelsea will no doubt be sorry to see him go. Many other observers, however, will be pleased, some of whom have always viewed Abramovich's ownership as a classic example of 'sportswashing'.
Having always insisted he was passionate about the club and had its interests at heart, he leaves as one of the most significant, controversial and influential figures in the history of English football.
Football finance expert Kieran Maguire
Chelsea will be a desirable football club for many given its status within the game. It's in London which makes it very attractive to ultra high net-worth individuals.
You have to caution that Chelsea has lost more money than any other club in the history of the Premier League and its stadium is a lot smaller than some of the other stadiums in London.
Speaking to some agents already today, I think the initial asking price is around £3bn. I think that is unlikely to be achieved. But if you are looking for a trophy asset - something to show off to your compatriots, to your friends, then Chelsea has an awful lot of attraction.
They are European and world champions - that does add to its allure.
There are three potential types of bidders we could see.
We could see the individual billionaire looking to add to their portfolio of well-known assets.
It could come for a private equity company based in the US who believes the riches of Premier League are undervalued in traditional markets.
We could see from a sovereign wealth fund as we've seen in relation to Manchester City, Newcastle and Paris St-Germain
This sale has come at a bad time in terms of the market. The share price of Manchester United - a good peer group company for Chelsea - is now trading significantly below its initial listing price in 2012. The market seems to have lost its love-in with football and this could impact on the ultimate price Chelsea is sold for.
24 February: Russian President Vladimir Putin launches a military invasion of Ukraine.
25 February: Labour's Chris Bryant tells MPs the UK government should remove Roman Abramovich's ownership of Chelsea and seize his assets, citing concerns about alleged links to corruption.
26 February: Abramovich announces he is handing "stewardship and care" of Chelsea to its charitable foundation.
27 February: Foundation members express concern over whether Charity Commission rules would allow them to run the club. Chelsea beaten 11-10 on penalties by Liverpool in League Cup final at Wembley after game ended 0-0.
28 February: Abramovich asked by Ukraine to help support their attempts to reach a "peaceful resolution" with Russia.
1 March: The Charity Commission says it is "seeking information" from Chelsea after the club was placed under the control of its charitable foundation.
2 March: Swiss billionaire Hansjorg Wyss claims he has been offered the chance to buy Chelsea from Abramovich.
2 March: Abramovich announces he intends to sell the club.
Former Newcastle and England striker Alan Shearer on BBC One: "It's a huge moment for Chelsea. He has brought massive success over his 20 years but it is not simple, selling a football club. It can take months or years.
"But more importantly, there is still no condemnation from Roman or the club about what is happening in Ukraine."
Former Manchester City and England defender Micah Richards on BBC One: "It is a better statement than Saturday, there is a lot more clarity and Roman Abramovich has done the right thing.
"It is so sad we have to talk about this on a football night after all the scenes we have seen. It is heartbreaking. The sooner it happens the better."