The Unite union has launched legal action on behalf of more than 1,800 Monarch staff who were laid off, in a move that could add millions to the taxpayers’ bill for the collapse of the airline. Unite, which represents engineers and cabin crew who worked for the airline, said the company had broken the law by failing to consult on redundancies.

It has also emerged that some staff were told to call a premium-rate phone line to hear news of their redundancy, with some billed almost £40 for the call. The administrators, KPMG, have since pledged to reimburse the costs of the calls.

Instead of the obligatory 45-day notice period, KPMG made 1,858 Monarch staff redundant on Monday. The accountancy firm was called in more than a month ago by Monarch bosses to consider whether the airline should be wound up, before it was put into administration on Monday morning.

Unite said it would be seeking additional compensation, which was likely to run to millions of pounds and, the union said, would be met by the government, since the airline is bankrupt.

The union’s national officer, Oliver Richardson, said: “The manner in which Monarch went into administration and the way the government allowed it to happen means there is a strong claim for compensation.” A similar case brought on behalf of 1,100 former Redcar steelworkers last year ended with a £6m payout.

The claim will add to the bill for taxpayers at a point when the government has paid £60m for the flight operation to repatriate stranded Monarch customers.

Although officials are holding talks with credit card companies, which may have otherwise faced claims from customers to reimburse flight costs, the Department for Transport was unable to give assurances that any money would be recouped. Asked if any commitment or indication had been given by card companies that they would assist, a spokesman said only that work was ongoing to help fund the costs of repatriating their customers.

He added: “These are costs that would otherwise have fallen to them to reimburse in many cases. Given the unprecedented demand on the aviation market Monarch’s collapse created, the government was forced to step in.”

Card payment companies may face the bulk of claims for reimbursement for 750,000 booked passengers yet to travel from the UK, as administrators said only 10% to 15% had Atol protection, although the CAA said that around half could be covered.

KPMG has agreed to refund the phone bills of Monarch employees who were charged as much as £40 to hear news of their redundancy, which the pilots’ union, Balpa, denounced as “unbelievably cold-hearted”.

An email sent in the early hours of Monday to all staff requested them to attend a meeting or contact a conference call at 9am, promising a further conference call for those unable to make the earlier event.

The second call was on an 0844 number that charged about 50p a minute on some mobile networks, leaving one Monarch pilot with a bill for £38.90 for the call.

Balpa’s general secretary, Brian Strutton, said the call and charges to staff were a “kick in the teeth” when they “were already down”. KMPG said it had asked a third-party provider to set up the conference call. A spokeswoman said: “We will ensure all participants are refunded any additional costs they incurred.”

The Civil Aviation Association said 23,321 Monarch customers had been returned to the UK, on more than 119 flights, by Wednesday morning. A further 11,000 were to return later on Wednesday.

Jet2 said it would add 100,000 more seats on flights from Birmingham, Manchester and Leeds Bradford airports this winter due to “increased demand” in the wake of Monarch’s collapse.