Jack Straw issued the warning after the home secretary’s idea – which would see the rising number of boats blocked before they can enter British waters – was branded “completely potty” by a defence official.
“I don’t think that just trying to push these people back is going to work,” said Mr Straw, who wrestled with the same problem 20 years ago.
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“It will only take one of these dinghies to capsize and everybody to drown – which is perfectly feasible – for there to be a hullabaloo, including in the Conservative party, and for the policy to have to be reversed.”
The tactic, modelled on the approach taken in Australia against migrants from Indonesia, could involve both the Navy and Border Force intercepting vessels after they leave France.
It was confirmed by Nick Gibb, the schools minister, who said: “We are looking at involving the use of boats in preventing boats from crossing the Channel.”
But a source at the Ministry of Defence also attacked it, saying: “We don’t resort to deploying armed force to deal with political failings.
“It’s beyond absurd to think that we should be deploying multimillion-pound ships and elite soldiers to deal with desperate people barely staying afloat on rubber dinghies in the Channel.”
Calling the idea “completely potty”, he also warned “it could potentially put people’s lives at even greater risk”, the Press Association reported.
More than 3,800 migrants have made the journey across the Channel so far this year, compared with 1,850 for the whole of last year.
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There are already fears of mass drowning similar to those in the Mediterranean, where 1,262 people died crossing from Libya to Italy and Malta last year.
The government says checks on fingerprint databases have shown that about 40 per cent of Channel migrants had made asylum applications in other EU states.
Ms Patel is planning tougher action after coming close to admitting that she is powerless to prevent crossings, saying: “I am working to make this route unviable.”
“This would involve “intercepting boats and returning those attempting to make a crossing,” she tweeted, but added that “we face serious legislative, legal and operational barriers” to making changes.
Migrants have threatened to jump overboard if they are turned back, in which case the Border Force must withdraw, because the preservation of life is paramount under the law of the sea.
Tony Smith, a former Border Force director general, said only an agreement with the French would stop dangerous crossings of the Channel.
“Once you’re on the waterways the law of the sea kicks in,” he told BBC Radio 4’s Today programme.
“Without a bilateral agreement on instant returns or joint patrols with the French, which would enable us to safely return them to France to be processed, we’re going to see, I’m afraid, continual numbers of this.
“And we need to find a way of breaking this circle and stopping the pull factor which is fuelling the smuggling supply chains.”