British Airways has averted the threat of a strike by cabin crew but still faces the threat of industrial action.
The GMB union is angry about BA's offer to reduce its pension deficit, saying its members have lost out in efforts to resolve the firm's pension problems.
GMB, which has 4,500 members at BA, is holding a consultative ballot on the pension reforms on 9 February.
Members are advised to reject BA's offer and if they do, a second ballot on industrial action will be held.
Members of GMB tend to work as ground or check-in staff and are among the lower-paid employees at BA.
"My members have had to take a disproportionate amount of the pain compared with the higher paid workers within the airline," GMB official Gary Pearce said.
Despite agreeing a deal on pay and sickness leave for cabin crew, BA remains under pressure over plans to cut its £2.1bn pension shortfall.
BA proposes raising the retirement age to 65, although staff can choose to increase their pension contributions from 5.25% to 8.5% if they want to retire aged 60.
If staff wish to maintain their current level of benefits and still retire aged 60 they may have to increase their contributions to as much as 11.5% to make up for falling accrual rates, according to trade union calculations.
The GMB points out that ground staff have a smaller proportion of their pay counted towards their pensions than cabin crew and pilots have.
Mr Pearce, who was involved with the negotiations, said he recognised that the pension scheme had a huge deficit and that staff would have to make some sacrifices.
But he denied that the union had accepted the company's proposals and said it had made it clear that it could not accept the reforms without consulting its members.
BA said it was disappointed by the position taken by the GMB.
"We have recently concluded 16 months of talks by accepting a collective proposal put forward by the four main trade unions at BA," the company said in a statement.
The T&G union has recommended its members accept the pensions offer, which would see BA pay £950m into the scheme in the next three years.
But T&G members have yet to vote on the proposals.