Brendan Barber called for new rights to be given to temporary and agency workers after highlighting a series of cases reported to the trade union movement.

In a speech on the opening day of the TUC Congress in Brighton, he said Polish workers were being paid as little as £2 an hour - well below the hourly minimum wage of £5.35 - for a 70-hour week, and not allowed any breaks.

He claimed one worker was sacked on the spot after asking if he could have a holiday, and a home worker, who was paid 60p for sewing a pair of trousers, had no rights to sick leave, holiday pay or redundancy protection.

"There are so many issues to tackle in this dark underbelly of British life," he told delegates.

Mr Barber said disreputable employment agencies were at the heart of many of the abuses and he called on the government to sign up to a planned European directive aimed at giving equal employment rights to agency staff.

"Now is the time to think again about employment agencies. It is time for another minimum wage moment - time to stop standing up for the bad agencies who undercut the good and threaten the reputation of all."

In a veiled warning to Gordon Brown, Mr Barber said it was "a critical time" for British workers, but also a crucial point in the political cycle.

Speaking ahead of the prime minister's address to the congress, Mr Barber condemned the treatment of public sector workers, saying they felt "battered and bruised", especially over pay.

With Mr Brown relying on affiliated party funding and union support in the event of a snap general election, Mr Barber made clear unions expected the government to "listen, consult and involve" workers more in future public sector reforms.

He claimed there was "too much top-down change, too many targets and too much faith that the private sector has all the answers".

"This year's centralised attempt to railroad through below-inflation public sector pay has been plain wrong and must never be repeated," Mr Barber said.

He called for "the unique public service ethos to once again take centre stage" and for unions to work with the government to shape a world-class service.

"To deliver the real improvements across our pubic services, we need to work together on the way forward," he said. "Not a retreat to the policies of the past, but a new way of doing things that delivers for service users, taxpayers and workers alike."