Eurotunnel has accused the Government of "distortionary and anti-competitive" behaviour over the award of contracts worth more than £100 million to provide additional cross-Channel capacity in the case of a no-deal Brexit.
Jacques Gounon, chief executive of the train operator's parent company Getlink, wrote to Transport Secretary Chris Grayling to voice "serious concern" about his decision to hand the work to three ferry companies.
Mr Gounon said Getlink "reserve all our rights to challenge such a measure both in the UK and France".
Mr Grayling has already come under fire for the award of one of the contracts - designed to ease the pressure on Dover if a no-deal Brexit causes delays - to a company with no ships which hopes to open a new route from Ramsgate to Ostend.
The boss of the port of Calais has branded the move "disrespectful", insisting his operations will cope with Brexit whatever changes it brings.
In his letter to Mr Grayling, dated January 2 and obtained by the Financial Times, Mr Gounon said Eurotunnel's Le Shuttle service was the "most efficient way" to supply vital goods to the UK and would remain so even if new border procedures were introduced after Brexit.
He said Eurotunnel was ready to take on extra work to smooth the flow of traffic across the Channel if it was threatened by a lack of public officials.
Eurotunnel and Dover 'highest priority'
A spokesman for the Department for Transport said 'maximising flows' through the Eurotunnel and Dover were a priority.
The company is working on the option of running additional freight trains and developing a rail terminal for unaccompanied trailers in Folkestone, he said.
Mr Gounon wrote: "In any eventuality, Eurotunnel remains prepared to deliver additional capacity under equivalent contracts to those you have signed with the ferry operators.
"I would ask you please to confirm that you are open to replicating the same kind of contracts with us."
A Getlink spokesman said the company would make no further comment.
A spokesman for the Department for Transport said: "The Government has invested heavily to ensure disruption at our ports is minimised in a no-deal scenario, while maximising flows through Eurotunnel and Dover continues to be our highest priority.
"The contracts agreed with ferry companies are entirely compliant with UK law and represent just one element of our sensible contingency work."