The Labour leadership frontrunners have been booed and jeered by union activists as they refused to condemn Tory plans for a £23,000 benefits cap.

Shadow health secretary Andy Burnham, shadow home secretary Yvette Cooper and shadow health minister Liz Kendall indicated they did not oppose the limit in principle during a hustings organised by the GMB in Dublin.

The rough ride came as the three confirmed their places on the ballot paper for the contest by recording support from at least 35 MPs. Figures published by Labour show Mr Burnham has been formally backed by 53 Commons colleagues since nominations opened this morning, ahead of Ms Cooper on 40 and Ms Kendall on 36.

Left-winger Jeremy Corbyn has been endorsed by 11 MPs, and shadow international development secretary Mary Creagh just five. Nominations are open until June 15.

The candidates faced questions over the Labour-union link, MPs' pay, the benefit cap and the party's election manifesto at the hustings.

Everyone except Ms Creagh said they would not share a platform with Prime Minister David Cameron in the run-up to the promised referendum on the EU. Many believe part of the party's implosion in Scotland was down to Labour politicians joining Tories at meetings and rallies during the independence referendum last year.

Mr Burnham and Ms Cooper were heckled by delegates for failing to give a yes or no answer when they were asked if they backed Conservative plans to reduce the cap on household benefits from £26,000 to £23,000 a year.

Both Ms Kendall and Ms Creagh said they were in favour of the change - to ensure it was impossible to receive more in welfare payments than the average family earned from work, though they warned it could cause localised problems.

To cheers, Mr Corbyn was the only one to declare himself opposed, insisting that in the absence of regulated private rents it would result in "social cleansing" in central London.

All five candidates said they did not believe Labour's election manifesto was too left wing, with Mr Burnham praising former leader Ed Miliband for the policies the party fought on.

Asked by Scottish delegate Duncan Walker how Labour could win back votes in Scotland, all five agreed the party needed to re-connect with voters.

Ms Cooper said Labour had to be a party for the whole country, adding: "We have to rebuild - but it will take us time."

The result of the election will be announced on September 12.