Stena Line will put on additional ferries from Scotland to Northern Ireland from Tuesday to help combat the impact of P&O suspending services, the UK Government has said.
Transport secretary Grant Shapps said the extra sailings will help retailers such as Marks and Spencer and Asda.
Sailings between Cairnryan and Larne have been cancelled for days and protests have been planned after P&O Ferries sacked 800 workers and suspended all of its services.
On Thursday, security guards boarded ships with handcuffs to remove fired crew, it was alleged.
Unions and politicians condemned the mass dismissal, blamed by the company on losses of £100m following the slump in travel because of the pandemic.
The Government has faced further calls to take legal action against P&O Ferries following reports that replacement agency workers are being paid less than £2 per hour.
The Rail, Maritime and Transport union (RMT), which represented many of the 800 staff fired without notice, said the crews were being replaced by Indian seafarers being paid 2.38 US dollars (£1.81) an hour.
General secretary Mick Lynch said: “The rule of law and acceptable norms of decent employment and behaviour have completely broken down beneath the white cliffs of Dover and in other ports, yet five days into this national crisis the Government has done nothing to stop it.
“These ships of shame must not be allowed to sail. The Government has to step in now and take control before it’s too late.”
P&O Ferries refused to comment on the pay of agency workers.
Shapps said he had not got “the details” of how P&O Ferries will recrew their ships but the Maritime and Coastguard Agency has been instructed to carry out safety checks of all vessels before they are allowed to sail again.
“I implore P&O Ferries to reconsider their decision. It’s not too late to acknowledge their mistakes,” Shapps told the House of Commons.
“I hope the reaction to that now infamous video in the House, the media and across the country tells them that this approach is quite simply acceptable.”
A spokesman for the company said: “Safety is the utmost priority for P&O Ferries and our crewing management partners.
“They have recruited high-quality experienced seafarers, who will now familiarise themselves with the ships, going through all mandatory training requirements set out by our regulators.
“Safety is paramount in our new crewing management model, which is used by many of our competitors and has been proven to be the most successful model in this industry and the competitive baseline.
“We will not be reducing crewing numbers. We don’t have a business if we don’t have a safe business.”