Big Four accountant KPMG is fighting to keep some of its biggest clients after it became involved in a corruption probe in South Africa.

In a growing scandal which has sunk disgraced PR firm Bell Pottinger, major customers of KPMG are demanding answers about its links with the controversial Gupta family whose businesses are alleged to have involved corrupt practices and money laundering.

KPMG audited an array of Gupta family businesses and faces a probe by industry regulators. Now it has come under renewed pressure as campaigners urge its customers to abandon it.

This includes fund house Old Mutual, one of the oldest names in the City and in South Africa. Last night it revealed it was in top-level discussions with KPMG about the controversy.

Other leading names urged to cut ties with KPMG include British American Tobacco, BHP Billiton, Standard Bank, Barclays Africa and Investec.

KPMG, which has a string of top-name clients, has launched an internal review, to be finished by the end of this month.

But the scandal is the latest to embroil KPMG which was auditor of Rolls-Royce, HBOS, Co-op Bank and football’s world governing body FIFA when all were mired in controversy.

KPMG is accused by campaign group Save South Africa of being a ‘central player’ in deals struck by the Guptas, who allegedly used close ties with president Jacob Zuma to win contracts for their businesses and influence ministerial appointments.

At one point KPMG reportedly audited almost 40 businesses for the family. It is being investigated by the Independent Regulatory Board for Auditors over its failure to raise the alarm as the Guptas diverted £1.7m of public cash to pay for a family wedding in 2013.

Four of the firms’s top executives attended, with one later describing it in a leaked email as ‘the event of the millennium’.

KPMG stopped working for the Guptas in March 2016 but has admitted it ‘should have stopped sooner’. Meanwhile, clients are seeking assurances that their reputations won’t be damaged by links to KPMG. An Old Mutual spokesman said yesterday: ‘Old Mutual is committed to doing business ethically and maintaining the highest standards of governance. We have continued our engagement with KPMG at the highest level on this matter.’

The calls for a boycott of KPMG came after PR firm Bell Pottinger collapsed. It faced an exodus of clients after it was found to have carried out a campaign in South Africa for the Guptas that stirred up racial tensions.

Campaigners have warned KPMG it could now be ‘the next Bell Pottinger’ if customers ditch it.

KPMG says it is taking the allegations against it ‘incredibly seriously’ and has brought in its international business to head an internal review of the South African division. It said: ‘KPMG is committed to being transparent about the results of this review. KPMG and the KPMG South African Board can assure the public, our people and our clients that the firm will accept and act on the findings of the review.

‘Should any wrong-doing be found, we are committed to driving absolute accountability and all required actions will be implemented.’