British Airways cabin crew will strike for a further two weeks in August, including over the bank holiday weekend, in a long-running dispute over pay and staff sanctions.

Crew in the lower-paid mixed fleet, working out of Heathrow on long and short-haul flights, are already in the middle of a lengthy walkout and will extend the strike to cover the period from Wednesday 16 August to Wednesday 30 August.

Unite, Britain’s biggest union, said the strike action would go ahead unless a deal was reached with BA on wages and on the sanctioning of striking workers.

Oliver Richardson, national officer at Unite, said BA was essentially blacklisting workers for taking lawful industrial action and described the offer to reinstate travel concessions for striking workers as “half-hearted”.

“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,” said Richardson.

“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.”

A spokeswoman for the airline said it was committed to flying all its customers to their destinations, by merging departures, leasing planes and crew from outside, and rebooking passengers on to other airlines.

She said: “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.”

On Wednesday BA apologised for a temporary problem with its check-in systems that caused long queues and delays at Heathrow and Gatwick. The disruption came about two months after more than 670 flights were cancelled because of a power failure over the spring bank holiday weekend.

The latest strike action signals more disruption for holidaymakers at a time when British passengers flying to and from continental Europe have been told to expect long queues at passport control because of tougher security checks.