With less than six hours before an unprecedented strike was due to begin at Heathrow, a temporary truce was reached between the airport management and the Unite union.

Talks to settle the bitter pay dispute between 4,000 workers including security guards, firefighters and engineers, and Heathrow, went on all weekend ahead of a strike that was due to begin at one minute past midnight on Monday morning.

On Sunday evening the industrial action planned for Monday was suspended – though a strike could still take place on Tuesday.

While the airport insists it is an excellent employer, the union says the massive profits Heathrow earns should be more equitably distributed – claiming that the airport’s chief executive, John Holland-Kaye, earns as much in two days as some of his employees do in a year.

Unite instructed its members to report for work on Monday, but still plans a walkout on Tuesday – as well as 23 and 24 August, the start of the bank holiday weekend in England and Wales, and two extremely busy days.

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The news came too late to reinstate all of the 100 or so flights that had been cancelled or diverted for today.

While British Airways says it will reinstate all the cancelled flights, the agreement came too late to “uncancel” all flights on other airlines, including Air Canada and United flights that were not dispatched from Toronto, Vancouver, New York, Washington DC and Chicago on Sunday night.

Passengers are entitled to be rebooked on alternative flights as close as possible to the original timing. Since the cause of current cancellations and any future disruption is not the airlines’ fault, there is no entitlement to a cash payout under European air passengers’ rights rules.

If Tuesday’s strike goes ahead as planned, more flights will be cancelled in advance – a strategy designed to limit the scale of the problem.

Heathrow is the world’s busiest two-runway airport, and has very little slack in the system. If travellers are delayed at security and arrive at the gate late, then the airline has to choose between keeping the plane on the ground – which will trigger problems later on in the day – or departing without some of the passengers on board, with all the problems that will create.

Passengers will be told arrive earlier than normal at Heathrow, and to check in all but a small shoulder bag or rucksack.There will be no charge for checking in what would, in normal circumstances, comprise cabin baggage.

Acas will also host continuing talks between British Airways pilots and BA’s management on Monday.

Nearly 4,000 pilots who work for British Airways and belong to the British Airline Pilots’ Association (Balpa) have voted overwhelmingly to reject a pay deal that BA says is worth 11.5 per cent over three years.

British Airways's A350 flight arrives at Heathrow

The two sides are set to talk on Monday and Tuesday, and the union has promised not to call a strike during negotiations. The law requires two weeks’ notice of industrial action, and no announcement of industrial action is expected before Wednesday – meaning the earliest action could begin is 20 August.

Meanwhile at Gatwick, 120 security staff working for the private firm ICTS are scheduled to strike for 48 hours starting at 6am on Saturday 10 August, which their union, Unite, says will “cause travel disruption”.

Gatwick airport says it does not foresee any delays or disruption to passengers.