By Nick Jones BBC News political correspondent

Anger and frustration which has been building up for months in the trade unions over the government's plans to step up the use of private finance in the public services finally exploded in ministers' faces at Labour's spring conference in Cardiff this weekend.

It is probably the most concerted union fightback since Tony Blair was elected in 1997.

Three of the biggest unions, Unison, the T&G and the GMB have combined forces to defend public services.

Delegates crowded into the rally to hear a vehement onslaught on the government's plans

They have been laying on regular publicity stunts at the conference to press home their opposition to privatisation, and the second day of the conference has seen some angry name calling.

Stephen Byers, the secretary of state for transport, hit back, telling the delegates it was "a battle between the reformers and wreckers" and he would not let "vested interests" stand in the government's way.

Afterwards he confirmed that he believed the wreckers included some union leaders. He was ready to take them on and explain to union members why change was necessary.

That angered the unions who had organised a rally at which nurses paraded with red heart cushions emblazoned with the words: 'Don't take the heart out of the NHS.'

Profit warning

Delegates crowded into the rally to hear a vehement onslaught on the government's plans.

Unison's general secretary, Dave Prentis, who in his conference speech had accused the "New Labour aristocracy" of getting too close to private companies, said that however the government tried to explain the private finance initiative it still meant that caring and essential services would end up being run for profit.

Bill Morris, the T&G leader, derided Friday's speech to the conference by the home secretary, David Blunkett, who suggested that ministers were not all powerful and should not take all the blame for failing services.

Labour Party officials seem to have been caught unawares by the furore

Mr Morris said the public had not backed Labour at the general election in order for Tony Blair to hand public services over to the private sector.

The need for ministers to heed the unpopularity of privatisation was underlined by John Edmonds, the GMB leader.

He said the government had invented its own poll tax and the policy of privatisation was imploding.

Unless Labour were careful they would be dragged down by it, just as the Conservatives were under Margaret Thatcher.

Labour Party officials seem to have been caught unawares by the furore that has engulfed the conference.

The unions believe they are beginning to force the government on to the defensive and that ministers are increasingly blowing hot and cold over what to do.

Worsening relations

Hostilities between the government and the unions have been at a low level since the 11 September attacks on the United States which de-railed the start of the unions' campaign to defend public services.

Relations have been getting worse for some time, which partly explains Tony Blair's praise for public servants a week ago.

But his ministers have indicated, to union fury, that it is still full speed ahead and it will place even greater focus on Mr Blair's closing speech to the conference on Sunday morning.

However, he's expected to make it abundantly clear that the unions wont be allowed to stand in the way of modernising public services.