Tony Blair has launched a strongly worded attack on Jeremy Corbyn after a new poll showed a groundswell of support for the left-winger.

The former prime minister said Mr Corbyn was “the Tory preference” to lead his party.

He branded the leading candidate’s left-wing supporters “reactionary” and said he would not want to win under a left-wing leader.

“[Labour] misunderstand the difference between radical leftism, which is often in fact quite reactionary – and radical social democracy, which is all about ensuring that values are put to work in the most effective way,” he said.

“Let me make my position clear: I wouldn’t want to win on an old-fashioned leftist platform. Even if I thought it was the route to victory, I wouldn’t take it.”

He later added: “Even if you did [win] it wouldn’t be right because it wouldn’t take the country forward, it would take it backwards. That’s why it’s not the right thing to do."

Speaking at the Progress think-tank Mr Blair said he would not endorse another candidate because it would “possibly not even [be] helpful for them”.

The first methodic poll of the Labour leadership campaign last night found that Mr Corbyn was on course to win the contest.

43 per cent of Labour supporters and members polled by YouGov for The Times newspaper said they would vote for Mr Corbyn in the first round of the contest.

Andy Burnham was in second place with 26 per cent, with Yvette Cooper third on 20 percent. Liz Kendall polled 11 per cent.

In the final round of voting Mr Corbyn would beat Mr Burnham by 53 per cent to 47 per cent.

Though polling such contests is difficult and the survey may not be an entirely accurate picture of the results, the figures do suggest that Mr Corbyn is doing very well.