The boss of Eurostar has said its trains between the UK and Paris are carrying 30% fewer passengers.
Chief executive Gwendoline Cazenave said with post-Brexit border checks and current levels of border staff, there were "bottlenecks" in stations.
Eurostar is currently running 14 services per day between London and Paris, compared with 18 in 2019.
Ms Cazenave said the company might not restore some services suspended last year due to the problems.
"The thing is now we are not able to run the same transport offer as what we had before in 2019, because of bottlenecks in stations," she said.
"We have a main issue in Eurostar terminals because of the new boarding conditions between the UK and EU, because of the impact of Covid, because of staff in the stations."
Ms Cazenave also said that Eurostar and both French and UK authorities were working hard on solutions such as having more border staff.
Last year, Eurostar announced it was halting its direct service from London to Disneyland Paris and also stopped services calling at Ebbsfleet or Ashford International stations.
It cited reasons which included financial problems due to losses suffered during the height of the pandemic and post-Brexit border checks - with more time needed to stamp UK passengers' passports
Asked if the services would be reinstated in the future, Ms Cazenave said: "We'll see, it depends on the way we can handle the big stations' issues."
She said the company's "objective" was to "be this backbone between big cities", such as London, Paris, Amsterdam and Brussels.
"These are the main cities, these are the main markets.... we are working for, which is our main role I would say," she said.
Currently, UK passengers travelling to the EU need their passport stamped when they cross the border, which has caused delays.
An Entry/Exit System, or EES, will replace the checks, but the technology has been delayed several times and is now due to be implemented at the end of 2023.
But concerns have been raised that initial registration for the system could cause delays to Eurostar services and lead to queues at the Port of Dover, as under the scheme people entering the bloc from non-EU countries - including the UK - will need to register fingerprints and a photo with their passport details.
Once travellers have given their fingerprints and details, that registration will be valid for three years. During that time it must be validated every time someone crosses the border.
Ms Cazenave told the BBC Eurostar was "pushing" for the system to be completely digital, meaning people could register details at home before they travel and it would "not be a bad customer experience".
"We know it's a big deal, we know it's a really big challenge," she said.
The Eurostar boss said the system would still work without "digitisation", but added it would need "a lot of investment, anticipation and staff.
On Tuesday, Eurostar announced its new brand which involved a merger between Thalys and Eurostar and said it hopes to carry 30 million passengers a year by 2030.