Members of the Unite union, who are already in the middle of a lengthy walkout, will prolong the strike from August 16 to the end of the month.
Unite national officer Oliver Richardson said: “Strike action will continue until the end of August unless British Airways hammers out a deal with Unite to resolve this dispute.
“The airline needs to get around the negotiating table and start recognising that punishing low-paid workers fighting for fairer pay is no way for a ‘premium’ airline to behave.”
The workers, who joined BA since 2010, were taking their 58th day of strike action on Thursday, and unless the deadlock is broken, the action will have lasted for the whole of July and August.
Unite said the action has forced British Airways to spend millions of pounds on leasing aircraft to cover the striking cabin crew.
The union described an offer by the airline to reinstate travel concessions for workers who have been on strike as “half-hearted.”
Unite is pursuing legal action on behalf of cabin crew who have faced “sanctions”, including the removal of bonus pay and travel concessions.
A BA statement said: “As we have done in previous periods of industrial action, we will ensure our customers reach their destinations.
“More than three months ago, Unite agreed that our pay deal was acceptable, but have since refused to ballot their members on it.
“Last week, we took the significant step of offering to return staff travel to crew who had been on strike, which was the biggest outstanding issue in the dispute, in order to bring the dispute to an end.
“Unite has now chosen to reject this offer and call yet more strikes.”
Mr Richardson added: “The offer to reinstate travel concessions for striking workers is half-hearted and fails to deal with the money British Airways has taken away from low-paid workers.
“In robbing striking workers of hard-earned bonuses the airline has sought to sow division and effectively blacklisted workers for taking lawful industrial action.
“Last week’s massive profits show British Airways can afford to tackle poverty pay rates and settle this dispute by dropping its divisive sanctioning of striking cabin crew.
“We would urge British Airways to start treating our members fairly and drop the bullying tactics to avoid the escalating cost and disruption that continued industrial and legal action brings.”