Strikes by hundreds of workers who assist Heathrow airport passengers with restricted mobility will intensify unless a pay dispute is resolved, a union has warned.

Unite said about 800 of its members employed by firm Wilson James are now taking industrial action, adding further walkouts could be held.

Some of those on strike staged a protest outside Heathrow’s head offices on Thursday calling on the airport’s management to intervene. Unite said its members were paid 10% less than Wilson James staff at Gatwick Airport.

A Wilson James spokesperson said: “We are disappointed that Unite members have chosen to take industrial action again despite efforts to resolve concerns collaboratively.”

About 500 workers began industrial action on 9 April and more than 300 colleagues have joined them since then.

The union said the strikes would intensify over the summer if the dispute was not resolved.

Unite general secretary Sharon Graham said: “Wilson James can well afford to put forward an acceptable offer.

“Heathrow bosses need to the tell the company to do just that, otherwise these strikes will continue to intensify with Unite’s full support.”

The Wilson James spokesperson said: “Our priority remains delivering a safe and reliable service for passengers, especially those who rely on our support.

“We have implemented contingency measures to minimise disruption at Heathrow and continue to value a constructive and respectful dialogue with our colleagues and their trade union representatives.”