Seafarers from abroad brought in to replace the 800 sacked British P&O Ferries crew are being paid as little as £1.80 an hour, unions have claimed.

The news emerged as Labour accused the government of doing “absolutely nothing” when it learned of the planned sackings, as a memorandum with the “game plan” of P&O was circulated on Wednesday evening.

The RMT union, which represents many of the staff who were abruptly fired last week by the Dubai-owned company, said Indian ratings brought in by offshore agencies to operate on the Dover-Calais route were being paid $2.38 (£1.80) an hour.

It is understood that P&O Ferries disputes the figures, but it declined to discuss the rates or give alternative rates as the crew are employed by a third-party agency.

The RMT general secretary, Mick Lynch, said it was “a shocking exploitation of those seafarers and another gut-wrenching betrayal of those who have been sacked”.

He called for sailings to be halted amid safety concerns over the wholesale replacement of P&O’s long-serving crew with fresh officers and ratings.

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.”

In the Commons, the shadow transport secretary, Louise Haigh, said the government had had the opportunity to stop the mass sackings. She said: “The truth is that P&O Ferries and DP World did this precisely because they knew they could get away with it … The impotent response from ministers shows that they were right to think that.”

She said the reaction to the memo from the Department for Transport was “the clearest proof that the government’s first instinct was to do absolutely nothing”.

Grant Shapps admitted he was made aware of planned redundancies on Wednesday evening at 8.30pm but assumed that they would be conducted in the same way as the 1,100 layoffs in 2020, through consultation, rather than “the cynical approach” used. The transport secretary said: “There is no excuse for the way in which it was carried out.”

Shapps also condemned P&O for offering crew redundancy packages “on the condition they sign non-disclosure agreements”. He said that the government would be reviewing all contracts with P&O and owner DP World, which is running two of Rishi Sunak’s new freeports. The UK’s development finance arm is also investing up to $720m with DP World in operations in Africa.

He was met with derision by Labour MPs after questions over P&O Ferries now staffing ships such as the Pride of Britain with foreign crew. Shapps replied: “To have a ship called the Pride Of Britain or Pride Of Kent … without having British workers I think would be completely wrong and I’ll be calling on P&O to change the name of the ships.”

Shapps said he had instructed the Maritime and Coastguard Agency to inspect all P&O ferries before they could return to service, including their operational drills to ensure that the new crew were properly trained, adding: “If they are not these ships will not sail.”

A P&O Ferries spokesperson said the company had recruited “high-quality experienced seafarers, who will now familiarise themselves with the ships, going through all mandatory training requirements set out by our regulators.”

He added: “Safety is paramount in our new crewing management model.”

The standard minimum wage in Britain is £8.91 an hour. Nevertheless, operators are not liable to UK regulation when they are sailing internationally and are flagged outside the UK. After Brexit, P&O Ferries reflagged some of its UK-registered ships to locations including Cyprus.

An update to the rules to ensure minimum wages are applied on ships for all nationalities of workers sailing between UK ports was brought in last year by the government, which would appear to mean P&O Ferries would have to pay the minimum wage to workers brought in on the Larne-Cairnryan route. However, cross-Channel ferries are exempt.

P&O would not confirm whether it paid the minimum wage.

No passenger sailings have operated since P&O Ferries made its shock announcement on Thursday, telling staff – many by video message – that they were being replaced immediately by cheaper agency crew.

P&O said services would remain suspended for several more days.

Protests were held on Monday afternoon outside the offices of DP World, P&O Ferries’ owner, and at parliament.