Lord West of Spithead said giving the navy command over the operation in the English Channel would backfire by providing a more “efficient conduit” for the work of traffickers.
Labour also accused Mr Johnson of trying to “distract” from Partygate after home secretary Priti Patel confirmed on Monday that she had asked the Ministry of Defence (MoD) to put the royal navy in charge of the operation to police migrant boats.
Sources told The Independent that initial talks with MoD officials about a new role in the migrant crossing crisis began only around 12 days ago – as Mr Johnson continued to face mounting questions about parties at No 10.
Lord West said: “This will not stop the migrant crisis. Picking them up at sea does not solve the problem of not giving them back. We don’t have an agreement with France to give them back yet.”
The peer, Britain’s first sea lord between 2002 and 2006, added: “All you’re acting as is a very efficient conduit for people smugglers, with migrants being picked up nice and safely by a royal navy warship.”
Defence officials also told The Independent there would be disquiet if the new duties became a long-term responsibility, although it is accepted that there is a need to help on a short-term basis. “People who would want to do that would join Border Force, not the navy”, said one officer.
It comes as the Telegraph reported that Ms Patel has been warned privately that in a worst-case scenario as many as 65,000 migrants could cross to the UK this year.
A Whitehall source told the newspaper: “It is not an estimate or a forecast. It's about how things work out on the other side of the Channel. If nothing happened and there was mass migration, what are the sort of numbers that we could look at?"
The decision to bring in the military is thought to be among a series of “Operation Red Meat” measures designed to please Tory supporters and attempt to recover from the prime minister’s plummeting poll ratings in the wake of the Partygate scandal.
Yvette Cooper, shadow home secretary, said: “This looks like Boris Johnson is using the situation to chase headlines to distract from the total mess he is in as a result of rule-breaking parties in No 10.”
Senior Conservative Tobias Ellwood, the Commons Defence Committee chairman, also said the plans were “rushed” and warned they could be a “massive distraction” for the navy. “This isn’t what our navy should be doing,” he said.
Lord West said tasking the military with tackling small boat crossings would prevent deaths, but not solve the crisis. The Labour peer told BBC Radio 4’s World At One programme: “I don’t think it makes any difference really to the problem of more coming across.
“Because if I was a people trafficker, I would say to them all, ‘Get in your little boat and go out there, the royal navy is now in control of all of the shipping that is looking out for you, you will be picked up and then you will be taken to Britain and then you’re pretty well on your way’.”
The royal navy taking charge of operations to halt migrant crossings would not mean the sudden appearance of warships or royal marines in the Channel, defence officials said.
The Independent understands that the royal navy’s role will not be to carry out “pushbacks” – the highly-controversial act of physically turning small boats around towards France.
A team under Rear Admiral Mike Utley, commander of the UK strike force, has been put in charge of “Operation Isotrope” – which will determine the contributions which the military may be able to make amid the rise in migrant boat crossings.
Defence officials say it is unlikely that a frigate or a destroyer will be stationed in the Channel. Royal marines or other navy personnel may, technically, be put on Border Force vessels, but military personnel have no powers of arrest, officials pointed out.
One fairly immediate option may be to ensure that the navy offers intelligence and technical expertise to Border Force.
The royal navy team could also, it is believed, look at a number of disused RAF bases, such as RAF Manston in Kent, or old military installations, such as Fort Blockhouse in Gosport, Hampshire, to see if they could be turned into residential centres for migrants while they are being processed.
Meanwhile, Ms Patel claimed that plans to operate pushback tactics in the Channel had been “well tested” and was still the government’s policy, although the measure is facing a series of legal challenges.
The home secretary also told the Commons the government was considering “all options” in moving asylum processing centres offshore after The Times reported plans are being drawn up to send migrants to countries such as Ghana and Rwanda for processing and resettlement.
Enver Solomon, chief executive of the Refugee Council, said using the military to “repel” those who seek sanctuary in the UK and looking to “expel” them offshore is "cruel and inhumane”.
A spokesperson for the MoD said that “unacceptable numbers of people continue to make the dangerous Channel crossings and last November’s tragic deaths serve as the strongest reminder of the need to stop them”.
The spokesperson added: “The government is exploring every avenue to prevent further crossings and detail of how that can be achieved will be made known in due course.”
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