A pension backlash is looming at IBM, trade union leaders have warned.

Unite, the UK’s largest union, said staff were increasingly angry that IBM was sticking to plans to close its final salary pension plan. IBM is also altering its early retirement scheme.

IBM should “brace itself for a backlash from thousands of employees”, the union said, after hundreds more IBM workers are understood to have joined up. Union meeting have been “packed to overflowing”, it said.

Around 5,600 staff are expected to be affected by the IBM changes, which will only see them receive what has been "accrued" so far, based on years of service and level of pay.

Unite said that typically people in their mid 50's could lose up to £200,000 as a result of the changes. It also expects between 700 and 1000 people to opt for early retirement before April 2010 when new early retirement provisions apply.

Peter Skyte, national officer at Unite, said the pension changes represented an “unacceptable attack” on staff. He added: “These highly skilled and experienced staff were key to the company's survival and they view the company's proposals as a kick in the teeth.”

IBM has large offices in London, Portsmouth, Winchester, Warwick, Greenock, and the North West.

The changes there follow a similar step taken by Fujitsu in May, after that company's scheme hit a £1 billion deficit.