The head of Liz Kendall's campaign has said Andy Burnham now represents the best chance of stopping Jeremy Corbyn triumphing in the Labour leadership election.
Toby Perkins, the MP for Chesterfield, said the complexities of the preferential voting system meant the shadow health secretary was the best hope of those who did not want the left-winger to seize the Labour crown.
His comments come after Yvette Cooper was seen to have boosted her chances with a powerful intervention calling for Britain to take 10,000 refugees from the Syrian conflict.
However Mr Perkins told The Observer that his team's canvassing returns suggested so many of Mr Burnham's supporters had put Mr Corbyn as their second preference, that Ms Cooper would struggle in a run-off with the Islington North MP.
"My analysis was that Andy Burnham would have a better chance of beating Jeremy Corbyn than Yvette Cooper if he was in the last two. On that analysis, I gave Andy my second preference, even though I have liked much of what Yvette has been saying," he said.
Under the alternative vote (AV) system adopted by Labour, voters can rank candidates from one to four.
If no candidate has 50% of first preference votes, the candidate who finished last is eliminated and their second preference votes distributed among the other candidates. The process continues until one candidate passes the 50% mark.
The Observer reported that canvassing by Mr Burnham's team - which mirrored the findings of Ms Kendall's camp - suggested that more than half his supporters had Mr Corbyn as their second preference.
This would mean that if Burnham was third behind Mr Corbyn and Ms Cooper, and the second preferences of his supporters were distributed between the two, Mr Corbyn would come out on top.
Ms Cooper's camp dismissed the claims saying the only independent polling by Yougov had shown the proportion of Mr Burnham's supporters who had put Mr Corbyn as their second preference was very similar to the proportion of Ms Cooper's.
"This outdated and tired argument that Andy's campaign have used that the only reason to vote Andy is because his second preferences are worse is just desperate and absurd," a spokesman said.
"Yvette's support has been growing and growing with every week that goes by because she is offering strong leadership and a radical and credible programme for government.
"Andy's support has dropped because he's not offering a clear or consistent alternative. He has lost first preference votes to both Jeremy and Yvette as a result."