Nicola Sturgeon has told the boss of P&O Ferries of her “utter disgust” at the treatment of seafarers who have been sacked and replaced with cheaper agency workers.

The ferry operator, which was bought by Dubai-based logistics giant DP World in 2019, cancelled sailings “for the next few days”, including on its route between Scotland and Northern Ireland, and sacked 800 of its staff.

Just hours after the company announced the move, the Scottish First Minister said she had spoken with its chief executive and had “made clear my utter disgust at this appalling treatment of its workers”.

She said on Twitter: “I made clear that @scotgov stands with these workers and will do everything possible to ensure fair treatment for them.”

At ports across the UK, workers who had been fired were refusing to leave ships on Thursday, leading to security guards with handcuffs being deployed to remove them.

The ferry operator insisted the decision to cut jobs was “very difficult but necessary” as it is “not a viable business” currently.

Earlier, Ms Sturgeon had hit out at the decision and tweeted: “Fire & rehire is an appalling practice & offends the basic principle of fair work.

“I’m deeply concerned at P&O announcement – due to the importance to Scotland of Cairnryan/Larne route obviously, but also the impact on 100s of workers.”

Scottish politicians from all sides in Holyrood also condemned the move.

Scottish Labour leader Anas Sarwar described the company’s actions as a “devastating decision for all the staff, their families and their communities, and an attack on trade unions”.

He added: “This will cost vital jobs, plunging people into unemployment in the midst of a cost of living crisis, and it will undermine pay and conditions for years to come.”

Scottish Green MSP Ross Greer said his party supports the RMT’s “efforts to save their members’ jobs, including by remaining on their vessels until this is resolved”.

He added: “On behalf of the Scottish Green Party, I offer the workers and their unions our solidarity and urge both the Scottish and UK governments to engage with the company and unions as a matter of urgency.”

Scottish Liberal Democracy economy spokesman Willie Rennie said it is “completely unacceptable for P&O workers to be sacked and replaced with foreign agency labour”.

Finlay Carson, the Scottish Conservative MSP for Galloway and West Dumfries which includes Cairnryan, said the redundancies are “devastating for the workers affected and are likely to have a huge impact in the local community”.

He added: “Reports that staff members were sacked in a pre-recorded video message demonstrate a shocking callousness if true.

“Not only that, but any disruption to crossings would have a hugely damaging effect on trade across Scotland – and of course in Northern Ireland.

P&O Ferries, which transports passengers and freight, operates three routes in addition to Cairnryan to Larne: Dover to Calais; Hull to Rotterdam; and Liverpool to Dublin.

A Scottish Government spokesman said it expected disruption to services directly serving Scotland on the Cairnryan to Larne route for around a week.

“There should be enough capacity to ensure that we can keep people and goods moving freely, but travellers should book where possible,” the spokesman said.

“We expect a similar position in relation to key export routes across the short straights to France.

“We will continue to monitor the position closely, and resilience partnerships stand ready to offer whatever support is required.

The spokesman said the Scottish Government was ready to support affected staff north of the border.

In a statement, the ferry firm said: “In its current state, P&O Ferries is not a viable business.

“We have made a £100 million loss year on year, which has been covered by our parent DP World. This is not sustainable.

“Our survival is dependent on making swift and significant changes now. Without these changes there is no future for P&O Ferries.”