Beer and sandwiches may be off the menu at Downing Street, but the trade union movement is back in the spotlight.

This week's annual congress, which begins in Brighton today, is likely to be the most politically charged for years.

It will be dominated by the Government's plans for more private sector involvement in public services, an issue that has put the TUC back at the centre of the political debate.

Tony Blair needs to deliver on his second-term promise to rescue ailing hospitals, schools and other public services and the Government's favoured method is by turning to the private sector.

It is a policy that has alarmed the left and public sector unions. A motion signed by 19 unions - a big enough block to ensure it will pass - will go before congress to put the case for the public sector.

The TUC's general secretary John Monks said the document proved unions shared concerns about the direction of Government policy.

It was also, he said, a forthright statement of the TUC's support for public services provided by public servants, although one that stopped short of ruling out any private sector involvement.

Speaking on the eve of congress, Mr Monks said unions wanted helpful dialogue with the Government rather than a war of words.

He said: "I am sure unions will have strong words to say in defence of the public service ethos and the public sector. There is no doubt public sector workers have felt under attack and under-valued since the unfortunate spin put on Labour's election manifesto.

"There are some issues on which clear disagreements with the Government remain. The tube is one of the most obvious.

"We will remain vigilant and will continue to make the case for quality public services.

"I hope that as we extend discussions with ministers then our reassurance will grow.

"Ministers need to be able to tell us that private sector involvement is limited and has clear justification other than an ideological preference for the private sector."

Senior union chiefs are less conciliatory, with the GMB's John Edmonds, one of Mr Blair's sternest critics, warning the Government it is playing with fire.

The Prime Minister is expected to try to damp down growing union anger by making a conciliatory speech to congress tomorrow and, possibly, flesh out his policy.

The part-privatisation of the London Underground through a £13 billion public private partnership (PPP) has angered unions.

Critics see the tube going the same way as the privatised railways and the Government's insistence on the private finance route is an indication of how it wants to modernise the rest of the public sector.

For major capital projects, such as hospitals and schools, private finance in one guise or another is seen as the only game in town although, theoretically, public money is still available.

Closing the Royal Alexandra Hospital for Sick Children and replacing it with a new building on the Royal Sussex Hospital site is being done through a £28 million private finance initiative (PFI).

Brighton's new library is also being built through a PFI. The long-awaited Peacehaven Community School, which opened on Friday, has been built using private finance.

Unions argue services will suffer as the private companies involved in projects seek to increase profits.

The length of the contracts, often for 20 or 30 years, and the cost of borrowing, more expensive for private companies than public bodies, also angers Government critics.

It is not just major building projects where these mini-privatisations are taking place. Private business are increasingly being encouraged to get involved in managing what has so far been a no-go area, schools and hospitals.

Again the railways are used as an example of how the council education departments and the health service could be fragmented - so-called Balkanisation - if private companies are allowed to bid for running different parts.

Contracting-out management of public services is hardly new but the pitfalls are obvious, nowhere more so than in Brighton and Hove's disastrous refuse collection service.

Andy Richards, who chairs the Brighton and Hove City Council branch of the public sector union Unison, concentrated on how staff transferred to private companies might be treated.

He said: "Private sector involvement tends to lead to downgrading of staff terms and conditions, longer hours, staff cuts, wage cuts. Often there is an anti-union bias with private companies as well.

"I think the experience has been, particularly in Brighton and Hove, that it has not improved services."

Whatever happens at the TUC this week sparks will fly and it will set the scene for when New Labour descends on Brighton for the Labour Party Conference in three weeks.