Heathrow faces two days of strike action next week after union workers rejected a revised pay offer by the airport.

A further two groups of staff at the London hub voted to go on strike in the ongoing pay row.

Unite said it would immediately enter into talks at the conciliation serve Acas on Friday evening.

Walk outs are planned for Monday and Tuesday unless the last ditch talks can resolve the dispute.

The latest two groups of workers – 100 Unite members in total – includes engineers, airside operation controllers, airfield transport officers and environmental officers.

If talks at Acas fail, it raises the possibility of these two groups of workers joining more than 4,000 others in a further two-day strike planned for the bank holiday Friday August 23-24, the union warned.

More than 170 flights have reportedly been cancelled by the airport over Monday and Tuesday. Heathrow confirmed that a number of flights would be reduced but that the airport would be operating wit contingency plans in place.

This came despite Unite warning Heathrow against choosing to pay millions of pounds in compensation to airlines for cancelled flights, instead of taking the “sensible option” of using that money to settle the dispute.

Unite estimates that about 88,000 passengers could be disrupted if 20% of flights are cancelled on both days of the strike action, amounting to compensation of £2,288,000 for the airlines affected.

The total bill could reach £4.6 million if a further two days of planned action on August 23-24 goes ahead.

The first two-day strike involving security guards, firefighters, engineers, passenger service operatives and passenger service drivers is planned to start at 00.01 on Monday, finishing at 11.59pm the following day (August 6).

Unite regional coordinating officer Wayne King said: “Heathrow faces a compensation bill in the region of £4.6 million from airlines if the planned strikes go ahead.

“Rather than provoking the disruption that strike action will cause, we would urge Heathrow airport to use this money for an improved pay offer that better reflects the hard work of the workers who keep the airport running safely and smoothly.

“This latest vote for strike action points to growing anger among the airport’s workers in a whole range of vital jobs which are essential to the smooth and safe running of Heathrow.

“Airport bosses need to heed this latest strike vote and the overwhelming rejection by our members of the revised pay offer which offers little over and above the original offer of £3.75 extra a day for many workers.

“It is in Heathrow bosses’ power to settle this dispute. We would urge them to work with us to do so and avoid the disruption to passengers that strike action will inevitably bring.”

Heathrow issued a reassurance to passengers, saying that the airport will continue to operate.

But it added: “We are working with our airline partners to consolidate and reduce the number of flights operating during the strike period.

“Unfortunately this does mean that some passengers will be moved to different flights by their airlines.

“This is a difficult time for Heathrow, our airline partners and, above all, our passengers, as the proposed strike action has been unreasonably designed to impact passengers taking well-deserved holidays.”

Which? Travel deputy editor Naomi Leach said: “Thousands of people will understandably be frustrated to have their travel plans thrown into chaos by these last-minute cancellations and sadly the disruption this August is set to continue.

“All passengers affected by these cancellations are entitled to a full refund or to be re-routed on to another flight to their intended destination – they may also be able to claim their money back via the chargeback scheme so should check with their card issuer.

“With ongoing disruption expected this summer, holidaymakers should contact their airline before travelling and allow extra time at the airport in case of delays.”