Describing the decision as unjustifiable, Prospect negotiator Ben Middleton said: "Having gone through one set of extensive and lengthy negotiations to resolve the existing two tier pay arrangements, the last thing we want is a further tier of benefits.

"Staff undertaking identical work will receive different rewards, which is clearly unfair and will almost certainly result in pressure on future wage claims."

A further concern among members, said Middleton, was that any move to a money-purchase scheme would place the responsibility for investment choices on the individual rather than more knowledgeable scheme trustees.

"We cannot see this proposal as anything other than an attempt to cut costs at the expense of BAA staff given that with only a 1.4% deficit, the scheme is in good health as far as the long-term is concerned.

"What the company describes as a more flexible approach to investment is really just another way of saying ‘this is the cheapest option for us’, where members are left to second guess the future performance of the stock market, despite having little or no experience of the market."

Prospect is seeking a meeting with BAA management to discuss improved terms for new starters and further assurances over the protected status of the existing scheme and its members.