Hundreds of air travellers face lengthy delays and cancellations after pilots employed by the airline TAP Portugal voted to go on strike next week over pay and conditions.

The airline is attempting to transfer customers to Heathrow, where it says operations are running normally, or to persuade passengers to consider alternative dates.

A spokeswoman for the airline said that problems could also be experienced on other European routes.

Around 150 of TAP’s 800 pilots are contracted to Portugalia. Last Saturday pilots from the same company took part in a 24-hour walk-out, but a spokeswoman for TAP said that disruptions were kept to a minimum.

She added that an agreement would probably not be reached between the airline and the union before next week, but said that the airline was hopeful that arrangements could be made to limit disruption to passengers.

Customers will be given the option of cancelling their flight and receiving a full refund, or to re-arrange their flight for an alternative date.

She urged concerned passengers to call TAP on 0870 601 0932 or to contact their travel agents.

Strike action will be suspended on Wednesday, when Manchester United travel to Porto for a European Champions League quarter final tie.

Portugalia pilots are planning a further one-day strike on June 13