Another P&O Ferries vessel has been detained, casting doubt on whether the firm will be able to restart the Dover-Calais route before Easter.

The Maritime and Coastguard Agency (MCA) said Spirit of Britain is not being allowed to sail after an inspection identified several safety issues.

P&O Ferries suspended services on its Dover-Calais route after sacking nearly 800 seafarers and replacing them with cheaper agency workers on March 17.

The firm said sailings would remain cancelled until Good Friday at the earliest, but the decision of the MCA means the restart could be delayed.

A spokesman for the agency said: “The Spirit of Britain has been detained due to surveyors identifying a number of deficiencies which were grounds for detention.

“We have advised P&O to invite us back once they have addressed the issues. We do not know yet when this will be.”

The agency did not specify what the issues were.

Pride of Kent – another vessel used on the Dover-Calais route – remains under detention.

The suspension of cross-Channel sailings by P&O Ferries means there is a shortage of capacity on the key Dover-Calais route.

This has contributed to large queues of lorries on the roads approaching the Port of Dover.

A 23-mile coastbound stretch of the M20 is closed from junction eight (Maidstone) to junction 11 (Westenhanger) to store thousands of lorries as part of Operation Brock.

The resumption of sailings by P&O Ferries boats is dependent on regulatory approval.

The firm has accused the MCA of carrying out inspections with “an unprecedented level of rigour”.

In response, the agency insisted it works “in exactly the same robust way” for every ship.

It previously cleared P&O Ferries’ Pride of Hull and European Causeway vessels, which are operating on the Hull-Rotterdam and Larne-Cairnryan routes respectively.

A P&O Ferries spokesman said: “The Spirit of Britain will remain berthed in its current port, following inspections by the Maritime and Coastguard (Agency).

“In the past few days, both the European Causeway and the Pride of Hull have been deemed safe to sail by the MCA, and we continue to work with all relevant authorities to return all our ships to service.

“We take the safety of our passengers and crew very seriously and look forward to all of our ships welcoming tourist passengers and freight customers again as soon as all mandatory safety tests have been passed.”