Downing Street is set to lift the 1% pay cap on public sector employees for both police and prison officers for the first time since 2013 after receiving new recommendations from several independent pay review bodies aimed at targeting recruitment and retention issues.
"Ministers are continuing to hold pay down, leaving professionals over £3,000 a year worse off".
Unison general secretary Dave Prentis said: "It's a tiny step in the right direction but not almost enough".
A final announcement on next year's pay deals will be made in the Budget in November.
The UK is facing a growing threat of coordinated strikes by public sector workers over the issue of the 1% pay cap. "There are no deserving or undeserving public sector workers", he said. But giving all public-sector staff an inflation-level rise would cost £5billion a year and wreck Chancellor Philip Hammond's plans to balance the books.
Four unions tabled motions to this week's Trades Union Congress in Brighton calling for some kind of nationally coordinated action, which were then included in a composite motion.
"There is a crisis in public sector pay and now is the time for action".
Ian Murray, of the Fire Brigades Union, said: "We all know that a Tory government will give working people nothing without a fight".
The POA prison officers' union said it was seeking industrial action over an offer which it said effectively amounted to a pay cut and would leave a majority of staff with rises of just 1.3%.
Mr Serwotka said PCS members had endured a 10% pay cut because of the cap, one that will rise to 20% by 2020 if it is not lifted.
Meanwhile, police chiefs warned that the pay award would put financial pressure on forces' already-stretched budgets and could impact on their ability to deliver services and avoid job cuts.
Labour leader Jeremy Corbyn said: "Let me be clear today".
Miss Truss said review bodies and departments were being given greater leeway to use pay to address "pinch-points" within public-sector staffing.
David Baker, of the Educational Institute of Scotland, said public sector workers should "show this government our collective strength". "They, like everyone else, deserve to have fulfilling jobs that are properly rewarded".
"Government will continue to ensure that the overall package for public sector workers recognises the vital contribution they make and ensures we can deliver world-class public services, while also being affordable within the public finances and fair to taxpayers as a whole".