A maritime union has urged the Transport Secretary to revoke P&O Ferries’ licences in British waters as the operator resumed services on one of its routes.

Nautilus International general secretary Mark Dickinson has written to Grant Shapps encouraging the Government to take urgent action against the ferry operator after it sacked 800 workers two days ago.

In a letter, published on Twitter, Mr Dickinson called for Mr Shapps to “hold P&O to account” in six ways, including revoking its licences to operate in British waters and pursuing “any legal option available” over how P&O handled the mass redundancies.

He wrote: “The action of P&O Ferries, terminating the employment of 800 British seafarers with immediate effect and without any consultation, is deplorable and a betrayal of British workers.

“This decision by P&O is a major blow to the British maritime industry… we cannot sit back and allow P&O Ferries to sink the nation’s maritime strategy.”

Mr Shapps wrote to Peter Hebblethwaite, chief executive of P&O, on Friday and said he was “questioning the legality of this move” and reviewing P&O Ferries’ contracts.

It comes after the ferry operator said it was cutting the jobs in a “very difficult but necessary” decision as it was “not a viable business” in its current state.

Sailings were halted on Thursday morning following the announcement, with P&O Ferries telling passengers they would remain suspended “for the next few days”.

However on Saturday, P&O said it had resumed one of its services crossing the Irish sea.

An update on the company’s Twitter feed said: “We have now resumed services between #POLiverpool & #PODublin. For booking information please visit our website.”

Other routes remain suspended or cancelled.

Shadow transport secretary Louise Haigh said the firm must not be allowed to resume sailings with replacement workers.

Ms Haigh said: “P&O must not be allowed to sail today with replacement agency labour for loyal workers unjustly sacked this week.

“Strong words from the Government are meaningless – they must step in and act.”

Meanwhile, the Rail, Maritime and Transport (RMT) union said around 300 people attended a demonstration near the Conservative Party spring forum in Blackpool.

RMT was in attendance at the protest, which was organised by Nautilus International.

East Hull Labour MP Karl Turner posted photographs of them carrying banners and flags which read: “Save P&O jobs! Save Britain’s Ferries!” and “Stop the P&O jobs carve up” as they paraded through the streets of the town.

Conservative Party chairman Oliver Dowden told Times Radio there is “revulsion” about P&O Ferries’ actions this week, adding that the Government is trying to establish whether the move was legal.

A spokesman for P&O Ferries said: “We took this difficult decision as a last resort and only after full consideration of all other options but, ultimately, we concluded that the business wouldn’t survive without fundamentally changed crewing arrangements, which in turn would inevitably result in redundancies.

“We also took the view, in good faith, that reaching agreement on the way forward would be impossible and against this background that the process itself would be highly disruptive, not just for the business but for UK trade and tourism.

“We have offered enhanced severance terms to those affected to properly and promptly compensate them for the lack of warning and consultation.

“The changes we’ve made bring us into line with standard industry practice.”

He added that all affected crew who were working at the time were notified face-to-face and in-person on board their vessels. Those who were off were called, emailed or texted.

A Department for Transport spokesperson said: “Ministers and officials have expressed their outrage and frustration at P&O Ferries on their decision and handling of their announcement, and are closely considering the department’s relationship with the company.

“The Transport Secretary has instructed a total examination of any contract in place with P&O Ferries and DP World across Government, and the department is working closely with unions, Department for Work and Pensions, and industry bodies to ensure that workers are supported and signposted to the most relevant support.

“We have serious concerns that their handling may not have followed the correct and legal processes, and strongly urge P&O Ferries to pause the changes announced, and speak with workers to repair the damage caused.”