Alexander Temerko, who has given the Tories more than £300,000 since 2012 and supports Boris Johnson, said he believed the London mayor would be the right candidate to reunite the party after a divisive EU referendum campaign.
Temerko is supporting the campaign for Britain to stay in the EU, in contrast to Johnson, who is one of the highest-profile leave campaigners.
However, he suggested the party needed a new leader regardless of whether Britain stays in or out of the EU, and endorsed Johnson as the best person for the job.
Writing for the Guardian, he said: “The Conservative party is fracturing following the resignation of Iain Duncan Smith and the infighting that’s likely to follow the referendum could be disastrous without any uniting influence.
“With a majority of just 12, seven to 20 rebel MPs are all it would take to cause a hung parliament and trigger another general election. My recommendation would be a leadership election immediately after the vote in June to unite MPs and members.”
He has previously offered to help fund any leadership campaign run by Johnson and described the London mayor’s main rival, George Osborne, as “very boring, very tedious, a very cold personality”.
“We may be on opposite sides of the referendum debate, but more than ever I believe Johnson offers the best hope of restoring unity to the Conservative party and of re-establishing the UK as a strong voice for openness and international cooperation,” he wrote.
“He can count on my backing – and hopefully that of other business-minded Conservatives – as the next party leader and our next prime minister.”
Cameron has said he plans to stay as leader whether or not he wins the EU referendum for the in campaign.
However, it is extremely likely he would face a leadership challenge if Britain votes to leave. Even if the result is a narrow victory for the remain camp, he could still face a bid to replace him as prime minister with a Eurosceptic more in tune with the Tory grassroots.
Ukrainian-born Temerko is a director of the energy company Offshore Group Newcastle (OGN) and a former executive of Russian oil companies.
He has personally donated about £330,000 to the Conservatives over the last four years and OGN has separately given about £470,000.
- This article was amended on 24 March 2016. An earlier version said Temerko had called for David Cameron to resign after the referendum. This has been corrected.
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