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Ed Miliband has said Labour is on the "right path" in supporting a cap on public sector pay rises, amid criticism from two of the party's union backers.

Unite and the GMB, two of the UK's largest trade unions, say the move is a "serious mistake" and could lead the party to a general election defeat.

But Labour's leader told the ´óÏó´«Ã½ he would "stand up for what's right" and it was "tough" if others disagreed.

Mr Miliband added that the party had to face up to "economic reality".

In 2010, the Conservative-Liberal Democrat froze public sector pay for two years for those earning above £21,000.

Then, last November, it said rises would be capped at 1% for each of the two years to 2015.

Labour's leadership announced at the weekend that this public sector pay restraint would continue if it regained power, given the economic situation.

But Len McCluskey, the general secretary of Unite - the UK's biggest union and Labour's largest donor - newspaper this would lead to the party's "destruction" and "certain electoral defeat".

He criticised what he called a "cavalier" shift in policy, arguing: "This confronts those of us who have supported Ed Miliband's bold attempt to move on from Blairism with a challenge. His leadership has been undermined as he is being dragged back into the swamp of bond market orthodoxy."

And the said it had seen a letter suggesting the GMB was considering its financial ties to Labour.

A spokesman for the union told the ´óÏó´«Ã½ that its general secretary Paul Kenny had written to senior Labour officials saying that the decision to back the public sector pay cap was a "most serious mistake".

Between July 2010 and last September, Unite gave £5m to Labour and the GMB provided £1.9m. This represented 43.2% of all donations made to the party during the period.

But, speaking to ´óÏó´«Ã½ political editor Nick Robinson, Mr Miliband said: "I don't seek a fight within my own party, but I do stand up for what's right.

"And if people don't like it, I'm afraid that's tough because I'm going to take the responsible path, the right path and the path which will show to the public that Labour is serious about the economic reality we will face if we come into government after the next general election."

Asked if he was urging public sector workers to agree to pay cuts to preserve jobs, Mr Miliband replied: "Absolutely. We're talking actually about a pay increase limited to 1%, but absolutely.

"Look, the priority now has to be to preserve jobs. I think that's a recognition that everybody would see around the country. We have got to do everything we can to preserve employment and as I say, this Labour Party is going to face up to those difficult choices we have to make."

But Bob Crow, general secretary of the RMT transport trade union - which is not affiliated to Labour - said the party was now "doomed" to electoral defeat.

Backbench Labour MP John Mann told ´óÏó´«Ã½ Radio 4's World at One there had "to be some proper thought on how government can reduce spending", rather than "by attacking the lowest paid". He said Mr Miliband needed a "credible plan" for government.

And fellow backbencher John McDonnell also accused his leader of "capitulation to Cameron's economic analysis", adding: "Len McCluskey's article sums up the general feeling among Labour Party supporters of overwhelming disappointment."

Unions founded Labour and supply around 90% of the party's funds. Mr Miliband narrowly won the Labour leadership contest in September 2010 ahead of his brother, David, after success with trade union members and affiliated societies pushed him into first place.