Crucial talks aimed at averting strikes by thousands of workers at Heathrow airport were adjourned on Saturday evening and will resume on Sunday, with time running out to prevent industrial action.

The airport has cancelled more than 170 flights on Monday and Tuesday because of the planned 48-hour stoppage by members of Unite.

Union leaders met Heathrow management under the chairmanship of the conciliation service Acas on Saturday after discussions until 11pm on Friday. After talks were adjourned on Saturday evening, a Heathrow spokesperson said: “Unite have rejected our revised pay offer today. While we are disappointed, we will continue to seek an agreement at Acas tomorrow.”

About 4,000 Unite members voted by almost nine to one to reject a pay offer Heathrow said was worth 7.3% over two-and-a-half years.

Wayne King, Unite’s regional coordinating officer, said: “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.”

An airport spokesman said: “Unite is proceeding with its unnecessary strike action on 5 and 6 August and we regret that passengers looking to get away on well-earned family holidays will be impacted by this. We have activated contingency plans, which will keep the airport open and safe on both strike days.

“We expect security queue times to be slightly longer than normal and advise passengers to check our website for detailed information on how to prepare for their flights and when to arrive at the airport. As part of our plans, we are working with airlines to proactively consolidate flights and rebook passengers on to alternative services in advance.”

A Heathrow spokesperson said: “Unite have rejected our revised pay offer today. While we are disappointed, we will continue to seek an agreement at Acas tomorrow.”

More strikes are planned on 23 August and 24 August if the dispute is not resolved.