John Prescott, Deputy Prime Minister, will make it clear at the annual TUC Congress in Blackpool tomorrow that Mr Mandelson should resist mounting pressure from employers and refuse to water down the Government's Fairness at Work White Paper.
The Confederation of British Industry is demanding that the Cabinet amend a series of key clauses favouring unions. The group is strongly opposed to a proposal to remove the limit on compensation for unfair dismissal.
Sources close to Mr Prescott say he believes the White Paper is a "damn good document" and will commend it to delegates, but by implication to his Cabinet colleague too.
The Trade Secretary is regarded as one of the union movement's principal enemies in the Cabinet.