Unite said today it was launching legal action on behalf of 1,800 Monarch workers who lost their jobs after the firm collapsed earlier this week.
The union, which represents around 1,800 engineers and cabin crew who worked for the airline, said it was lodging employment tribunal proceedings over a failure to consult on redundancies.
Unite national officer Oliver Richardson said:
Through no fault of their own, former Monarch workers are out of pocket and out of a job.
While, understandably, a lot of the focus is on passengers, Unite is determined to ensure that Monarch workers, who worked so hard to try and turn the airline around, are not left high and dry.
He added: "The manner in which Monarch went into administration and the way the government allowed it happen means there is a strong claim for compensation by former Monarch workers."
Separately, the British Airline Pilots' Association (Balpa) said it will be seeking compensation "for the shabby way our Monarch pilots were notified of their company's demise and their own sacking".
It has around 400 members affected by Monarch's collapse, and said staff had been given a premium rate number to dial into to hear the redundancy news.
A spokesperson for the joint administrators at KPMG said a key priority was to make sure all Monarch staff were communicated with "as quickly and as openly as possible" following their appointment at 4am on Monday morning.
They said KPMG employees were present at crew rooms at the five airports Monarch operated from by 4.30am, so pilots and aircrews could be personally made aware of the administration, while all employees were invited to one of eight face-to-face meetings held at 9am, with a conference call for those unable to attend at 2pm.
Later on in the afternoon of our appointment, the administrators' team held meetings at Luton with the company's HR team and the government's redundancy payments office, to ensure that over the following 48 hours, employees receive the documentation they need to accurately make the claims they are entitled to make.
All such letters were sent to employees last night, on day two of the administration, which is extremely fast for any administration, let alone one of this scale.
The administrators' team and the retained company HR team will continue to provide whatever assistance employees require in order to accurately submit these claims.
On Monday, Monarch collapsed into administration after bowing to pressures including global terrorism and intense competition. A rescue operation costing £60m was launched in order to fly back 110,000 passengers stranded abroad.
Administrators KPMG confirmed said that 1,858 employees at Monarch Airlines and Monarch Travel Group had been made redundant.
Other airlines have already shown an interest in taking on Monarch employees, with EasyJet saying earlier this week it had reached out to flag opportunities in Luton and Gatwick.
Balpa general secretary Brian Strutton said: "Since Monday we have already lined up potential job opportunities with 18 different airlines and we have arranged our flight crew futures event to take place on 17 October at Gatwick, and we encourage all Monarch pilots to attend."