A Swedish businessman who lent £2.6m to the Conservatives is to leave the party to become an adviser to Gordon Brown.
Johan Eliasch will carry out a review on green issues for Gordon Brown
Johan Eliasch had resigned from his post as Tory deputy treasurer but the party had said he remained a supporter.
He has accepted the prime minister's invitation to conduct a review on deforestation and green energy.
Mr Brown was accused of "deeply cynical" manoeuvring by Tory MP Sir Malcolm Rifkind - but the PM denies any party political motive.
Mr Eliasch, who spent £8m to preserve 400,000 acres of Amazon rainforest, will not renew his party membership when it lapses next month.
'Not party political'
He said in a statement he was looking forward to the task, adding: "There is a universal agreement that climate change must be addressed so this is not a political party issue and therefore I shall not be a member of any political party during this important work."
Earlier this week, Shadow Chancellor George Osborne said Mr Eliasch was a "committed Conservative" who had resigned to spend more time concentrating on combating climate change.
Asked about Mr Eliasch's appointment, Conservative leader David Cameron said: "Johan Eliasch is a great expert on green and environmental matters and he has been asked to do a job by the government on a non-party basis.
"If this is a genuine attempt to involve experts then it is a good thing, if it's low politics it's a bad thing."
Asked about the difficulties of repaying such a large loan, he said he had "radically broadened the base of the Conservative Party", so it was not too reliant on large individual donations.
BBC Political Editor Nick Robinson said Mr Eliasch had complained privately that he felt Mr Cameron had abandoned his commitment to occupy the political centre ground.
But Mr Cameron has said there will be no "lurch to the right" and has dismissed critics of his reforms within his own party as "blasts from the past".
Since he became prime minister in June, Mr Brown has said he wants a government of "all talents" which is "built on consensus" - and has offered several posts to members of opposition parties.
Earlier this week two Tory MPs, John Bercow and Patrick Mercer, and Lib Dem MP Matthew Taylor agreed to advise the prime minister on their various areas of expertise.
Mr Brown's spokesman said on Friday there had been no party political motive behind Mr Eliasch's appointment to the unpaid role.
"The basis upon which he has been appointed is his expertise in relation to climate change and deforestation," he said.
But former foreign secretary Sir Malcolm said Mr Brown was "dangerously close" to abusing his position.
"It is perfectly clear that his over-riding motivation in a number of recent appointments of both Conservatives and Liberal Democrats has not been the unique expertise of the individuals involved but the fact that they belong to a different political party," said Sir Malcolm.
"It is deeply cynical. Gordon Brown promised a new honesty in politics. We are not seeing it."
Mr Eliasch was one of four major donors to the Conservatives questioned by police investigating "cash for peerages" claims.
A former chairman of the Young Conservatives, his wealth comes from Head, the Dutch sports equipment company, which he runs from London.
He founded the charity Cool Earth which encourages people to donate money to buy tiny sections of the rainforest to save them from destruction.