P&O Ferries has replaced the UK crews it sacked without warning last week with Indian seafarers paid just 2.38 US dollars (£1.81) an hour, a union has said.
The replacements are being paid well below the minimum wage in the UK, The Rail, Maritime and Transport union (RMT) said.
General secretary Mick Lynch said: "The news that the seafarers now on ships in British ports are to be paid 2.38 dollars an hour is a shocking exploitation of those seafarers and another gut-wrenching betrayal of those who have been sacked.
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"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."
The minimum wage in the UK for people aged 23 and above is £8.91 per hour.
But companies using UK ports often register ships in other countries, allowing them to pay lower wages. For example, some vessels operated by P&O Ferries are flagged in Cyprus.
When the firm informed staff on Thursday they were losing their jobs, it told them it was aiming to halve crewing costs. No sailings have operated since that date, the PA news agency said.
The company posted on Twitter on Monday morning that services will remain suspended "for the next few days".
Demonstrators will gather outside the London offices of P&O Ferries owner DP World on Monday afternoon, before marching to Parliament.
Also on Monday afternoon, Labour will force an emergency vote in the Commons demanding that the Government takes action to outlaw the so-called fire and rehire of staff, which involves making workers redundant before giving them their jobs back with worse terms and conditions.
The party will also call on the Government to suspend contracts with DP World until the matter with P&O Ferries is resolved.
Labour Leader Sir Keir Starmer said: "What I want to see is those workers reinstated to their jobs, as they should never (have) been dismissed from them.
"I want to see the Government take action here to ban this fire and rehire, which is the cause of this in the first place.
"There's a vote on that this afternoon. So, what I don't want to hear is ministers, Government ministers, complaining about what P&O have done.
"Now they've got the chance to do something about it, vote this afternoon for reinstatement and banning fire and rehire."
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."
