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.
Some 36,105 of the 247,339 cast in that section did not count.
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.
A Labour Party spokesman said: "These ballots were not counted because the affiliate members who returned them did not tick a box to confirm that they support the Labour Party and not another political party.
"The ballot papers carried clear and prominent instructions that people's votes would not be counted if they did not do this."