Agents will face the nightmare of finding available seats to rebook clients at peak holiday time if the British Airways strikes extend into the summer.

Talks on Tuesday failed to produce a deal and the Unite union threatened a fresh ballot to take the strikes into July.

“A summer strike would be horrendous,” said agency manager Karen Hinz, of Bath Travel in Christchurch. “We’ve managed to rebook everyone up until now, but there won’t be availability in July and August. We’ll have people shouting at us.”

Cabin crew completed a 17th strike day on Thursday and were set to start a fresh five-day stoppage on Saturday until Wednesday, June 9. BA promised to ramp up its schedule from Saturday, claiming more staff “want to work”.

It aims to operate 80% of Heathrow long-haul flights and 70% of short-haul. It said services out of Gatwick and London City would continue as normal.

A deal remains possible, but the strike will end only if BA restores the cabin crew’s travel concessions it has withdrawn, or Unite gives up on their restoration.

The strikes will halt for the World Cup. The England football team flew out this week with Virgin Atlantic, while BBC pundit Gary Lineker launched BA’s campaign for the UK to host the 2018 tournament.