Mr Brown has met both Mr Obama and Mr McCain this year
Gordon Brown has rejected suggestions he endorsed Barack Obama's US presidential candidacy in an article.
The prime minister, who praised the Democrat's policies in the piece, said he had "admiration" for both Obama and his Republican rival, John McCain.
Mr Brown told a press conference it was for the American people to decide on their next president.
The Tories have said Mr Brown should explain the comments in his article and not be seen to be "taking sides".
British prime ministers traditionally do not back one candidate over another.
In the article for The Monitor, a Parliamentary magazine, Mr Brown wrote of concerns about the housing slump, adding: "Around the world, it is progressive politicians who are grappling with these challenges.
"In the electrifying US presidential campaign, it is the Democrats who are generating the ideas to help people through more difficult times. To help prevent people from losing their home, Barack Obama has proposed a Foreclosure Prevention Fund to increase emergency pre-foreclosure counselling, and help families facing repossession."
He did not mention Mr McCain.
Asked about the comments at a press conference earlier, Mr Brown said: "The decision on the American election is a matter entirely for the American people and I have scrupulously met both Senator McCain and Senator Obama and talked to them both about the issue that affect our two countries and the future of global issues."
He said he had met both in Washington and London and had enjoyed his conversations with them adding: "I'm a great admirer of both of these distinguished senators" and he was "proud" to know them both.
Shadow foreign secretary William Hague, for the Conservatives, said Mr Brown should not have written the comments.
He said: "A responsible British prime minister needs to be ready to work with either presidential candidate after the US election, and should neither take sides nor be seen to be taking sides.
"Gordon Brown needs to make clear why he appeared to be favouring the Democrats in this article and to explain whether this was his deliberate intention or a careless mistake."
But Downing Street denied that Mr Brown was taking sides in the US election.
A spokesman said it was simply "an article written ahead of the party conferences in Britain" which "talks about some of the measures being taken around the world by centre-left political parties to deal with the current global economic challenges".