For the first time in a generation, a Labour prime minister is set for a showdown with the unions at the TUC Conference.

"We think PFI has failed. It is a failed experiment and we will continue to present the evidence of this to the government. We believe public services should stay public

Unison

After years of sitting on the political sidelines, the movement is in open revolt over Labour's plans to use private companies to rescue Britain's ailing public services.

If, as expected, Tony Blair addresses this year's conference, delegates will want to hear more than bland assurances that the NHS is in safe hands.

Union membership

Unison - 1,272,470 TGWU - 858,804 AEEU (engineering and electrical) - 728,211 GMB - 683,860 MSF - 404,000 USDAW (shopworkers) - 310,222 CWU (communication workers) - 284,422 PCS - 267,644 NUT (teachers) - 206,100 GPMU - 200,676 Source: TUC

Despite a series of high-level meetings at Number 10, the TUC is threatening mass demonstrations against the government plans to extend the scope of private initiatives in the provision of public services.

They are questioning whether private profits will lead to the public gains that the government says the involvement of private companies will provide.

Industrial action

The anger of the unions is enhanced by the fact that a majority of TUC members now work in the public, not the private sector.

Unison members are prepared to strike over privatisation of services

The largest union now is Unison, which represents public sector workers across the local authority and health care sector.

Unison leader Dave Prentis has promised industrial action at all 29 of the hospitals currently being built under the Private Finance Initiative (PFI), if the so-called privatisation of jobs continues.

He has also predicted that there will be "two years of conflict" with the government over public-private partnerships.

Biggest public sector unions

UNISON GMB TGWU NUT

Mr Prentis plans to use the TUC conference to rally support for Unison's campaign against the 'creeping privatisation' of public services.

He is counting on the support of the big four unions to swing the TUC behind the campaign.

The Transport and General Workers Union, which also includes a sizeable number of public sector workers, and the GMB are expected to back Unison's motion calling for a halt to PFI.

But the AEEU engineering union, whose leader Sir Ken Jackson is close to New Labour, is not expected to support the Unison motion - and has insisted on a compromise motion which recognises the role of the private sector.

In which case, the battle is likely to continue at this year's Labour Party conference.

Body of evidence

The unions claim to have amassed a considerable body of evidence which shows that PFI does not represent good value for money.

"We think PFI has failed. It is a failed experiment and we will continue to present the evidence of this to the government.

"We believe public services should stay public." A Unison spokeswoman told BBC News Online.

But she added: "There is nothing wrong with private companies building a hospital.

"What we think is wrong is when they take over the running and management of clinical services."

Party funds

However, even this conciliatory tone may be at odds with many union activists.

Unison members voted at its annual conference in June to review its �1.3m annual donation to Labour party funds.

Increasingly, Unison members are asking why we hand over millions of pounds of members money to fund a party which is attacking our jobs, wages and conditions

Unison resolution

The union is the party's biggest single donor.

The resolution, passed by a slender majority and against the wishes of the union leadership, said: "Increasingly, Unison members are asking why we hand over millions of pounds of members money to fund a party which is attacking our jobs, wages and conditions."

Although it is unlikely that the union will cut off Labour funds altogether, the message to the party's leaders is clear.

The 'rank and file', as union members used to be known, are not going to take public-private partnerships lying down.

Earlier this year, the GMB cut its donation to the Labour Party by �1m and has openly discussed making donations to the Liberal Democrats.

The fire service union supported candidates from other left wing parties at the general election, which it felt were more in tune with its views.

Whatever the outcome of this year's TUC Conference, the trade union movement's relationship with the party it founded more than 100 years ago is clearly changing.

Balance of power

It no longer has the political clout it once had, but membership is increasing for the first time in decades (although at 7.3 million, it is less than half what it was in the 1970s).

The unions themselves have also changed.

As they have become smaller, they have become more white collar.

Last year, for the first time the number of professionals in unions outnumbered blue collar workers.

Biggest union

At the same time, the balance of power within the TUC has shifted decisively in favour of the big public sector unions.

Many of our members are part-time female workers and they are worried about their conditions as much as their pay

Union official

Unison has taken over the mantle of Britain's biggest union from the Transport and General Workers Union.

The strength of Unison's powerbase in the public sector has made it largely immune to the changes going on in the rest of the movement.

Its members are very often low-paid, unskilled manual workers, who look to their union to perform the traditional role of protecting jobs and getting them a better deal in the workplace.

They worry about what happens when services formerly run by the NHS and local councils are transferred to private contractors.

These are the very people Mr Blair is relying on to bring about the improvement in public services he promised at the general election.

He risks upsetting them at his peril.

Pension rights

Employment law dictates that when jobs are transferred into the public sector most terms and conditions are protected.

But these can be eroded when job descriptions change - and new workers are hired on lower wages.

Private companies are also thought more likely to sack workers - and less likely to give part-time workers pension rights.

One Unison official told BBC News Online: "Many of our members are part-time female workers and they are worried about their conditions as much as their pay.

"For example, we don't know of any private scheme that offers pension rights.

These PFI companies couldn't make a profit if they gave their workers pensions. It is simple as that."