Union leaders have warned that some staff at IBM Global Information Services could be up to £6,000 a year worse off under the proposed changes to overtime and call-out rates, which were announced internally last month.
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Staff working in the Warwick, Manchester and Portsmouth offices of IBM's strategic outsourcing service delivery business are understood to be hit by the changes.
"People are very angry," said Peter Skyte, national secretary of the Amicus MSF union. "There have been limited negotiations and changes to take home pay affect people's lifestyles and mortgages."
IBM declined to comment.
Last year union leaders accused IBM of offering its IT professionals a miserly redundancy package, as it consulted its workforce on hundreds of possible job losses as part of a global restructuring.
The terms of IBM's redundancy package for UK consultants are significantly less generous than redundancy deals from other major IT suppliers, leaders of the Amicus IT Professionals Association claimed.
Since then, IBM has declined to reveal how many UK redundancies it has made, adding that it was constantly "re-balancing" skills and resources.