UK-based partners at Grant Thornton, the accountancy firm, have voted to back the sale of a majority stake to Cinven, the private equity firm.

Sky News has learnt that GTUK’s partnership voted unanimously to support the deal, in a move expected to be announced internally as early as Monday.

Support for the deal had been widely anticipated within the firm, given that it will crystallise substantial windfalls for its 240 partners.

The transaction, which is understood to value GTUK at up to £1.5bn, is one of the most significant ever struck in Britain’s audit sector.

Cinven saw off competition for the deal from New Mountain Capital, which owns a big stake in Grant Thornton’s US business.

Other contenders, including EQT, Permira and Carlyle, were eliminated earlier in the process.

GTUK has improved its financial performance following a turbulent period for its leadership, with a £1.3m fine being imposed for “serious failings” in 2022 in relation to its audit of Sports Direct, the sportswear empire founded by Mike Ashley and now known as Frasers Group.

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It was also handed a £2.3m penalty the year before for demonstrating a “serious lack of competence” in relation to its work on Patisserie Holdings, the owner of the collapsed cafe chain Patisserie Valerie.

Since then, Grant Thornton has slashed the number of so-called public interest entity (PIEs) audit clients, a category which includes banks, insurers and other companies deemed to be of particular importance.

A spokesman for GTUK declined to comment on Sunday.