First Minister Nicola Sturgeon joined UK ministers in condemning the move by P&O’s Dubai-based owner DP World, which dismissed the staff via video message “with immediate effect”.
Passengers were urged not to make non-essential journeys on the company’s Cairnryan-Larne route because of "very limited space” on Stena Line’s services between nearby Loch Ryan Port and Belfast.
UK shipping minister Robert Courts told MPs P&O had said all its services would be halted for “approximately a week to ten days while they locate new crew”.
Sailings on its routes from Dover to Calais, Liverpool-Dublin and Hull-Rotterdam have also stopped.
P&O told its seafarers in the video message “your employment is being terminated with immediate effect” because ferries would be “primarily crewed by a third-party crew provider” to “drastically reduce costs” following annual losses of £100 million over the past two years.
The firm tweeted on Thursday: “Where possible, we are organising travel via an alternative operator.
"Space is very limited, so we would suggest, if your journey is not essential, please do not travel today."
The operator said the cancellations were because of “a programme of work to become a more competitive and efficient operator, providing a better service to our customers across the tourism and freight industries”.
“While we enact these changes, there will be significant disruption across P&O Ferries services over the next few days,” P&O said.
A Stena Line spokesperson said: “We operate six sailings per day on our Cairnryan to Belfast service and there is currently additional capacity on most sailings.
"If that additional capacity reduces in the days ahead, Stena Line would like to remind passengers that it has a number of sailings on other routes crossing the Irish Sea from Birkenhead (Liverpool), Holyhead and Fishguard.”
Transport Scotland said it expected disruption on the Cairnryan route “for around a week”.
Its spokesperson said “There should be enough capacity to ensure that we can keep people and goods moving freely, but travellers should book where possible.”
The disruption came as political and union fury with P&O’s sudden announcement mounted.
Ms Sturgeon tweeted: “I have just spoken directly to the CEO of P&O and made clear my utter disgust at this appalling treatment of its workers.
"I made clear that @scotgov stands with these workers and will do everything possible to ensure fair treatment for them.”
Scottish Greens MSP Ross Greer said: “P&O should be universally condemned for this attempt to sack their entire UK workforce and replace them with agency staff on worse pay and conditions.”
Mr Courts told the Commons: “These are hard-working, dedicated staff who have given years in service to P&O.
"The way they have been treated today is wholly unacceptable.
"Reports of workers being given zero notice and escorted off their ships with immediate effect, while being told cheaper alternatives would take up their roles, shows the insensitive way in which P&O have approached this issue – a point I have made crystal clear to P&O’s management.”
Mr Courts said he had told chief executive Peter Hebblethwaite of his anger at the way the company had treated its workers.
Labour’s shadow transport secretary Louise Haigh described the company’s action as a “betrayal of the workers that kept this country stocked throughout the pandemic”.
“There are images circulating of what we are told are handcuff-trained security, some wearing balaclavas, marching British crew off their ships,” she said.
“This is not a corporate restructure, it is not the way to go about business.
"It is beneath contempt – the action of thugs.”
Commons transport committee chair Huw Merriman said unless P&O reinstated its staff, “it’s hard to see a way back for them commercially”.
He said DP World “need to understand the British public will not do business with companies which treat their employees with such contempt”.
Susannah Streeter, an analyst at investment firm Hargreaves Lansdown, said: "The nature of the redundancy announcement is unlikely to help it win back customers.
"It could fast turn into a severe reputational headache for the company, with a big union fight looming ahead.’’
The Rail, Maritime and Transport union has instructed its members to stay on board and said there had been reports of security guards boarding ferries in Dover to remove crew.
General secretary Mick Lynch said: "We are seeking urgent legal action and are again calling for the [UK] Government to take action to stop what is fast turning into one of the most shameful acts in the history of British industrial relations.
"If this happens at P&O, it can happen anywhere and we are calling for mass trade union and wider public mobilisation and protest against the company."