Employers were warned last night not to use the recession as an excuse to water down staff pensions after Wm Morrison became the third blue-chip company this week to take the axe to its final salary scheme.

Britain’s fourth-biggest supermarket operator said that it was altering its defined benefit scheme so that the 10,000 employees in it would clock up future pension benefits based on their career-average pay, and not their final salary.

The move was announced 24 hours after Barclays said that it was closing its final salary pension scheme for 18,000 long-serving staff — something that other clearing banks are expected to follow it in doing — and 48 hours after BP said that it was closing its final salary scheme to new recruits.

The biggest union in Lloyds Banking Group, the LTU union, has demanded the bank guarantee that it will keep open its final salary pension for the scheme’s existing 30,000 members.

Morrison’s new arrangements start on August 1, but Richard Pennycook, finance director, emphasised that pension benefits earned in respect of past work would not be affected.

He said that the reform was as much about reducing risk for Morrison as about the short-term cost savings, although the grocer will book an exceptional credit of £50 million to £70 million as a result.

Marc Bolland, the chief executive of Morrison, insisted that it had consulted staff fully before its move. He added: “We are not the only ones doing this.”

Derek Simpson, joint general secretary of Unite, the union, criticised the step, saying: “It’s outrageous and totally unacceptable for employers with good long-term prospects to use the recession as an excuse to cut pensions.”

The Morrison scheme, which was closed to new recruits in 2003, has £2 billion of assets. At its worst, its deficit neared £300 million. Morrison began consulting the trustees a year ago and has injected £200 million into the scheme.

Morrison said that like-for-like sales in the 13 weeks to May 3, excluding fuel, were up by 5 per cent on a year earlier. This beat City forecasts and the shares rose 6½p to 253¾p.

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