Details deal were not disclosed, but the Labor party and the union admitted a deal was struck in June, The Times of London reported Thursday.

The party's mounting debts forced it to approach various lenders for "reassurance" on future financial backing, The Times said.

Britain's Electoral Commission has said details do not need to be disclosed, as they describe future transactions.

"Political parties do not have to tell the commission about any future funding arrangements," the commission said. But, the commission suggested this summer that funding guarantees should be disclosed.

"If a political party can only keep going with guarantees from outside backers those guarantees should be fully and publicly disclosed as loans or donations," said Liberal Democrat Treasury spokesman Mathew Oakeshott.

"Even if they are operating a millimeter within the law, Labor are clearly breaching the spirit of the law," he said.

Unite has donated $20.7 million to the Labor party since Prime Minister Gordon Brown took office, the Times said.