Extra Brexit border checks at the Eurostar terminal in London have led to longer queues and capacity has been cut by nearly a third.
Jacques Damas, the outgoing chief executive, said that limiting capacity on some trains is the only way to long queues at London St Pancras International are only being avoided by the company limiting capacity on some trains.
He said even with all border booths manned, the station can only process a "maximum" of 1,500 passengers per hour compared with 2,200 in 2019, before the Brexit transition period ended.
In a letter to Tory MP Huw Merriman, who chairs the Transport Select Committee, Mr Damas said that the stamping of British passports by foreign border officials adds "at least 15 seconds" to the processing time for each passenger.
Mr Damas also said that Eurostar took out loans of £500 million because of the drop in income caused by the pandemic.
He told Mr Merriman that these are the "main factors" behind the company deciding not to resume operations at either of its Kent stations - Ashford or Ebbsfleet - until at least 2025, and will stop direct trains between London and Disneyland Paris from June next year.
Serving the Kent stations would "make things even worse" at London St Pancras International as it would "take away from London vital border police".
Mr Damas wrote that it must focus on its core routes - connecting London with Paris and Brussels - where it can "charge higher prices to our customers", rather than "pursue a strategy of volume and growth".
He also stated that the European Union's new Entry/Exit System (EES) "hangs over us".
EES is due to be introduced next year, and is expected to involve travellers from non-EU countries such the UK having their fingerprints scanned and a photograph taken to put them on a database the first time they enter a member state.
Mr Damas wrote: "In such an uncertain context, I have the duty, as chief executive, to make the right decision and secure my company's future. I am therefore cautious not to overcommit."
Gwendoline Cazenave will take over from Mr Damas on Saturday