British Airways cabin crew will mount picket lines at Heathrow on Thursday at the start of a three-day strike over pay.
The walkout by members of Unite in the so-called mixed fleet crew, which is made up of workers who have joined the airline since 2010, follows a 48-hour stoppage last week.
BA said it will operate all its long-haul services to and from Heathrow during the strike and cancel a small number of short-haul flights.
Unite claims the mixed fleet crew are on "poverty" pay rates, with many forced to take on second jobs or turn up for shifts if they are sick because they cannot afford to be off ill.
BA has said its pay offer for mixed fleet crew was consistent with deals agreed with Unite for other British Airways workers, reflected pay awards by other companies and will ensure rewards for mixed fleet remain in line with those for cabin crew at its competitors.
Unite said more than 800 mixed fleet cabin crew workers have joined the union since the start of the dispute, taking membership to more than 2,900.
Unite general secretary Len McCluskey and shadow chancellor John McDonnell, whose constituency covers Heathrow, will join pickets on Thursday, said the union.
The union's national officer Oliver Richardson said: "British Airways needs to drop its confrontational stance which is causing so much anger and leading to plummeting morale among its mixed cabin crew.
"With British Airways' parent company forecasting massive annual profits of around £2.3 billion, it is clear the airline can afford to recognise the hard work of its mixed fleet cabin crew by paying a proper decent wage.
"Rather than trying to bully workers and focusing its resources on leasing aircraft to cover striking cabin crew, British Airways should focus its energies on trying to resolve our members' legitimate concerns over poverty pay.
"Unite urges British Airways to wake up to the deepening anger of mixed fleet cabin crew and start valuing their contribution by meaningfully engaging with Unite to address poverty pay."
BA said: "We will operate all our long-haul services to and from Heathrow and all services to and from Gatwick, London City and Stansted.
"We will merge a small number of our short-haul services at Heathrow, resulting in the cancellation of only 1% of our total scheduled flights across the three days. Customers affected will be able to fly slightly earlier or slightly later."