John Monks: Warning of "aggressive" conference

Mr Monks said there were still concerns over exactly what Mr Blair would introduce in his second term.

This was despite several statements from Mr Blair that private sector involvement in health, education and other services does not mean privatisation.

Mr Monks told BBC News 24's One to One programme he could foresee "a very difficult period of relationships" between the union movement and the government.

"The government won a fantastic majority for the second time, nobody's ever done that in the history of modern British politics, and yet the atmosphere I'd say was flat," he said.

"The particular area that's most concerning the TUC is what are the boundaries that are between the public and the private sector in the delivery of public services.

"There is a strong reaction on the public-private issue around the trade union movement... and there's more aggression in the relationship than there's been, well since Neil Kinnock started to change Labour round with a lot of union support."

Blair: Private sector involvement does not mean privatisation

Mr Monks' warning follows several clashes over the party's plans to have private sector finance and management involved in the public services.

Disquiet has been expressed from all sections of the party, including backbench MPs and grassroots membership.

Unions have said it is the number one issue facing the party.

Last month, the GMB union said it would slash up to �1m from its contributions to the party over the next four years, in protest.

In June the UK's biggest union, Unison, voted to review its financial links with Labour - worth �1.3m a year - over the issue.

Health union Unison is reviewing its donations to Labour because of the issue

Lord (Roy) Hattersley, Labour's former deputy leader, has warned that Mr Blair and the unions are heading for a "confrontation" over the matter which could bring huge disruption to services.

But ministers have insisted the concept is based on the need to provide quality services and value for money for taxpayers.

One to One will be broadcast on BBC News 24 on Saturday 25 August and repeated on Sunday.