Theresa May's leadership campaign received £15,000 from a major Tory donor who asked to be withdrawn from David Cameron's resignation honours list.
Ian Taylor was among supporters who contributed a total of £275,000 to Mrs May's leadership campaign within two weeks, according to the register of MPs' financial interests.
The Vitol oil boss was reported to have been nominated for a knighthood by Mrs May's predecessor as PM.
He later said he did not want his name to go forward amid a croynism row over Mr Cameron's choices for honours.
Mrs May has declared 18 donations from a two-week period linked to her Tory leadership campaign, which came to an abrupt end last month after Andrea Leadsom withdrew from the head-to-head contest.
Other donations include £20,000 from IPGL, of which Michael Spencer is listed by Companies House as a director and chairman.
It has been reported Mr Cameron wants to secure a peerage for Tory donor Mr Spencer although The Times claimed this was in doubt following opposition from the Lords Appointments Commission linked to his company Icap's role in the Libor scandal.
The register also shows distribution firm Sun Mark, whose chairman Rami Ranger backed Remain during the EU referendum, donated £25,000 to support Mrs May's leadership bid.
Pro-Europe electrical cable manufacturer Tratos (UK) gave £5,000.
Mrs May has faced questions over her desire to deliver Britain's exit from the EU given her support for Remain during the referendum campaign.
But the PM has adopted a "Brexit means Brexit" mantra before negotiations with the EU begin.
On what will happen to unspent money from the leadership campaign, a spokesman for the Prime Minister said: "Surplus money raised but not spent will be donated to Conservative campaign headquarters, as per the rules set out by the 1922 Committee."
Mr Taylor declined to comment.
He previously donated £500,000 to the Better Together campaign to reject Scottish independence in the 2013 referendum.
At the time the campaign faced calls to return the money, with then-Scottish first minister Alex Salmond raising concerns about Vitol, which was heavily fined over its role in the Iraq oil-for-food scandal.
Mr Taylor has donated hundreds of thousands of pounds to the Tories in recent years.
Controversy over Mr Cameron's choices for honours was sparked by reports that he had recommended knighthoods for four pro-EU cabinet colleagues - Philip Hammond, Michael Fallon, Patrick McLoughlin and David Lidington.
The Sunday Times reported that Mr Cameron also requested a Companion of Honour award for George Osborne, who was dismissed as chancellor by Ms May.
Will Straw, head of the failed official pro-Remain campaign, was proposed for a CBE, and more than 20 Downing Street staff were recommended for awards, according to the report.