A senior union official has urged new Labour leader Ed Miliband not to "forget in five minutes" what he has heard from workers during the past five months of campaigning.

Len McCluskey, launching a country-wide tour as part of his campaign to become general secretary of Unite, Labour's biggest affiliate, said the party could now show that it stood on the side of working people to face the "big challenges" ahead.

"Working people are angry - they've been forced to make all the sacrifices during the private sector recession. Now they know there is more pain coming as the coalition Government takes an axe to public spending.

"My message to the coalition Government is clear - punish working people and they will punish you in return. You will pay at the polls for listening to the business lobby rather than catering to working people and pensioners.

Mr McCluskey's comments come as it emerges that nearly 10 per cent of votes cast in the Labour leadership election did not count. More than 36,500 of the 375,000 ballots were declared spoiled by officials.

The overwhelming majority of those came in the trade unions affiliates section - which was crucial in allowing Ed Miliband to defeat elder brother David by a margin of just 1.3 per cent.

Many of the votes are believed to have been ruled out because trade union members omitted to tick a box confirming they were Labour supporters.