Gate Gourmet has begun an investigation into union allegations of improper corporate activity and "major health and safety concerns" at the company.
The union wants all 670 sacked Gate workers to be re-employed
The caterer, which is at the centre of the recent industrial action at Heathrow Airport, said it did not believe there had been any wrongdoing.
News of the investigation marks an apparent escalation of the dispute between Gate Gourmet and the union.
Gate sacked 670 of its staff last week, sparking a British Airways walkout.
The caterer removed the workers after a dispute over cost-cutting and work restructuring.
British Airways, which is Gate's main client at Heathrow, then saw its own ground staff at the airport conduct a two-day wildcat strike in sympathy with the catering employees.
The union representing the sacked Gate workers - the Transport and General Workers Union (T&G) - is continuing negotiations with the company to try and get all 670 staff reinstated.
The allegations of improper corporate activity and heath and safety issues at Gate were brought up by T&G boss Tony Woodley during Wednesday's talks with the company bosses.
The T&G would not give any further details about the allegations but insisted they had "no bearing on either the origins or the resolution of the present industrial dispute".
But Gate accused Mr Woodley of "threatening" to take the allegations to British Airways unless the caterer immediately rehired all the sacked employees.
T&G boss Tony Woodley made the allegations
"[Gate chairman] David Siegel [then] immediately called a senior British Airways executive to inform him of the situation," the caterer said in a statement.
Gate added that it was hiring a team of forensic accountants to investigate the allegations.
"We don't believe that there has been any wrongdoing," Mr. Siegel said in a statement, "but we intend to make certain that this is the case."
A T&G spokesman said the union simply wanted Gate to investigate the claims.
The union says it will help Gate with restructuring and cost-cutting only if it first re-employs all the sacked staff.
In a separate development, British Airways has opened an investigation into the illegal walkout by its own staff.
It called last week's wildcat strike a "body blow that defies belief".
The firm also vowed to find out whether workers were intimidated into joining the action which led to the cancellation of all its flights.
BA baggage handlers, bus drivers and cargo workers were among the ground staff who walked out.
The strike resulted in the cancellation of 700 flights, causing disruption to the journeys of 100,000 travellers and costing BA more than £30m.