Trade unions may be acting unlawfully if they call further strikes over Birmingham City Council’s controversial salary shake-up without putting an improved offer to a formal ballot, a senior politician warned last night.

Alan Rudge, the human resources cabinet member responsible for implementing the single status pay and grading review, said it was "doubtful" whether unions could rely on a show of hands at workplace meetings as a mandate for continued industrial action.

He was speaking hours after new contracts for 40,000 council employees came into force, marking the start of a slimmed-down seven-grade wages system.

But the largest Council House union insisted Coun Rudge was mistaken in his interpretation of the law and threatened further walk-outs in the run-up to the May local government elections.

David Hughes, the vice-chairman of UNISON in Birmingham, said the council had created a "legal quagmire" by imposing the pay and grading review even though 16,000 union members are refusing to sign their new contracts.

Many of the 24,000 staff who have accepted the new working arrangements did so under duress because they feared they would be sacked if they declined to do so, Mr Hughes said.

The new arrangements are designed to outlaw pay inequalities between men and women and end distinctions between blue and white collar jobs.

Almost half of the workforce will be better off, but more than 4,000 employees stand to lose pay - in some cases more than £10,000 a year.

The unions staged a one-day strike at the beginning of February but called off a further stoppage in order to hold negotiations with Coun Rudge.

An improved offer, reducing the number of people losing money under the new system, was rejected by nine meetings of union members.

Coun Rudge is insisting the revised offer must be put to the council’s 20,000 union members in a formal ballot.

He said: "It is ridiculous to suggest people were under duress to agree the new contracts. If that was the case, no one would have signed.

"What we have offered is fundamentally different to the original proposals. We have told the unions they must put it to a ballot, private and confidential and held in the proper manner.

" It is a doubtful legal issue as to whether they are entitled to have industrial action without a further ballot."

But Mr Hughes indicated that an additional ballot would not be held. He added: "Thousands of council workers attended nine mass meetings to consider the council’s offer and every meeting voted overwhelmingly to continue to take industrial action.

"Each meeting received a detailed presentation on the council’s offer and members decided for themselves that it fell well short of what was acceptable.

"Union members have a legal right to continue to take industrial action to improve the offer and resolve outstanding contractual issues."

Claims by Coun Rudge that the mass meetings were infiltrated by extremists and representatives from the Socialist Workers’ Party were dismissed by Mr Hughes as "the latest ridiculous outburst by a politician who is in denial of his own responsibility for this dispute".

He added: "Trade union members have a variety of views across the political spectrum from left to right, but the union members who attended the meetings were united in their opposition to the council’s proposals.

"The council claims a majority of the workforce have signed the new contracts, but this has only happened under duress as many workers believed they would be dismissed. We are aware that many of those that have signed also voted to take industrial action to get them changed."

Areas still in dispute, according to UNISON, include the number of staff facing pay cuts and the size of those cuts, a proposal to introduce performance related pay and the arrangements for appealing against re-grading decisions.