The politician responsible for introducing a new pay and grading system for 40,000 Birmingham City Council employees has issued a "keep talking" plea to unions in an attempt to avert strike action.

Alan Rudge said his door remained open and hinted that he might be prepared to compromise over the scrapping of bonuses for blue collar staff, which could leave some manual workers up to #10,000 a year worse off.

Coun Rudge (Con Sutton Vesey) said he was happy to look at "anomalies", where certain employees might be hit unfairly, and there was also the possibility of "rejigging" job descriptions in order to amalgamate bonuses into salary. He would not steamroller through the changes.

However, he made it clear that there would be no back-tracking on the general principles behind the pay and grading review – to end unfair wage discrimination between men and women and blue and white collar employees.

The impact of the pay shake-up, agreed by the cabinet on Monday, will see 42 per cent of council staff receive a pay rise and 12 per cent suffer a pay cut. The remaining 46 per cent will continue to receive their existing salary.

Losers under the new system are to have their existing pay protected for three years, but this does not include controversial weekly bonuses paid regardless of performance, which will disappear.

Overtime for weekend working will also be scrapped.

Unison, the largest council union, is balloting its membership for strike action. Other unions may follow suit.

Union leaders said it was unacceptable that 5,000 council staff will lose money under the new system.

Coun Rudge promised a separate review of the bonus system, adding: "Everyone regrets that this has to be done and we realise there are anomalies that could crop up.".

But he warned: "Some of the bonuses are totally indefensible. Some, if structured correctly, wouldn't be bonuses and might require rejigging job descriptions.

"We will look very carefully at anything put to us by the unions and take it into account. But if a bonus is not part of the core salary you can't put it into protection. That's the law.

"We are in continuing negotiations with the unions about what comes after bonuses. But we are not going to keep outdated and ineffective bonus systems."

A council hardship fund set up to help those worst hit by the salary changes will offer loans to employees.

Applicants will have to offer collateral against the loans, which Coun Rudge said could be in the form of property, pension plans, endowment policies, shares, or "anything of value".

Coun Rudge said he regretted Unison's decision to hold a strike ballot, adding that most union members stood to benefit from the pay and grading review.

"The vast majority of the 42 per cent who stand to gain are Unison members and are being treated fairly for the first time. The unions should be very pleased that this is taking place and that inequalities are being removed."

In a direct plea to the trade unions, Coun Rudge added: "I hope people keep talking to me. I will keep talking back. We certainly don't want small issues which we could deal with to get in the way of reaching agreement.

"But we have to make a move to deal with this issue. There has to be acceptance of the principle that change has to be made."