The union said it would lodge employment tribunal proceedings over the company's failure to consult the workers on redundancies, and said the employers had not given the necessary notice or statutory pay.
The airline declared bankruptcy on Monday after failing to secure fresh capital or sell the business, leaving British authorities racing to rescue tens of thousands of customers stranded overseas, while Monarch staff were made redundant.
The union, which represents 1,800 Monarch engineers and cabin crew, said workers are entitled to a statutory 45 days notice under United Kingdom law, and that employees with two years or more of service should receive redundancy pay.
On Wednesday it was revealed Unite the union would begin legal action on behalf of more than 1,800 Monarch airline workers who lost their jobs.
He said the union could not say how much this was likely to cost the taxpayer.
Almost 2,000 employees have been made redundant as a result of the collapse of travel firm Monarch, which also saw thousands of flights and holidays cancelled.
Brian Sutton of BALPA 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".
"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".
"That is why Unite is doing everything it can to assist former Monarch workers in securing new jobs, offering free legal advice and launching legal action to secure the compensation they are owed, as well as helping members find jobs with other airlines".
"Since our appointment, one of our key priorities has been to speak to all of the companies' employees across all locations and provide them with the support and assistance they need at this distressing time", Nimmo said.
A total of 34,608 people have already arrived back in the United Kingdom on flights arranged by the Government and aviation regulator the Civil Aviation Authority (CAA). "We will ensure all participants are refunded any additional costs they incurred".
Eighty staff in Bromley have been made redundant as Monarch airline and tour fell into administration. As of an announcement yesterday, 23,321 customers had been flown to the United Kingdom on 119 flights.