The Competition Commission has warned the Channel tunnel operator's ownership of MyFerryLink may be anti-competitive.
Eurotunnel bought three ships from operator SeaFrance after it went into liquidation to run the service.
The company said it would challenge the provisional findings of the Competition Commission.
P&O Ferries and DFDS also run ferries on the Dover to Calais route, but the commission said it believed it would be better if there were two independent ferry companies competing with the tunnel.
SeaFrance, which was owned by French state-owned rail firm SNCF, went into liquidation in November 2011. Eurotunnel bought three of its four ferries for Â£52m last year.
The commission claimed Eurotunnel, which now runs 16 ferry crossings a day, bought the boats to prevent Danish rival DFDS from picking them up cheaply and driving down prices.
The commission said Eurotunnel may have to sell or dispose of the business to address its concerns.
Conservative MP for Dover and Deal, Charlie Elphicke, welcomed the report.
"Clearly (Eurotunnel) should sell the ships," he said. "They should go back to running the tunnel, which they do very well.
"They should get out of the ferry market and they should allow open competition without them trying to get too much market share."
Eurotunnel accused existing operators of using the commission process to protect their own interests.
It said its new brand, MyFerryLink, increased competition and brought more choice for customers.
"Eurotunnel intends to continue to work with the Competition Commission to allay the concerns raised by existing ferry operators and to demonstrate that the creation of MyFerryLink is a good thing for the market as it is both pro-customer and pro-competition," said chief executive Jacques Gounon.
The Competition Commission is expected to publish its final report on 14 April.
Passengers and freight firms could face higher fares as a result of Eurotunnel running its own ferry services between Dover and Calais, a watchdog has said.