French interior minister Gerald Darmanin said the UK must uphold international maritime law.
Speaking on Thursday morning he said: “France will not accept any practice that goes against maritime law, and will not accept any financial blackmail.”
Tensions between London and Paris over the migration crisis have surged in recent days, with Britain threatening to hold back £54million in funding.
On Wednesday Ms Patel confronted her French counterpart, telling him the British public “expect to see results”.
Over 14,000 migrants are believed to have reached Britain by boat so far this year, a record figure.
On Monday alone 785 made the trip, the second highest figure for 2021.
However, there are warnings legal issues mean the “push back” tactic may never be used.
Lucy Moreton, from the Immigration Services Union, warned the UK would need French permission.
Appearing on BBC Radio 4’s Today programme she said: “In practical terms, if this happens even once, I would be surprised.
“There are understandably a lot of constraints around it and you cannot do this with a vessel that is in any way vulnerable and more importantly you need the consent of the French to do it.
“Because when you turn the vessel back toward France, when it is across the median line it has to be intercepted and rescued by the French and it appears the French will simply not engage with this, in which case it’s – if you excuse the pun – dead in the water.”
Conservative MP Tim Loughton, who sits on the Commons home affairs select committee, agreed the proposal is “not going to happen”.
Speaking to the Daily Mail he commented: “t sounds good. But I’m afraid in practice it’s just not going to happen.
“These are flimsy boats coming over. Even those that are tougher are completely weighed down.
“Any boat coming up alongside at speed would capsize most of these boats anyway and then we’re looking at people getting into trouble in the water and drowning and then we’ll get blamed for that.
“It sounds good pushing them back but it’s not going to work in practice.”
Mr Darmanin held talks with Ms Patel on Wednesday, at Lancaster House, during a G7 meeting.
The Home Secretary told her French counterpart, reducing the number of migrant boats crossing the channel is her “number one priority”.
A Government insider commented: “The Home Secretary was clear with the French interior minister that the British public expect to see results.”
However Mr Darmanin has already turned down a UK request to create a joint French-British command centre in northern France, to tackle the migrant boats.
In July the Government pledged to give France another £54million to help stop channel crossings.
Speaking to MPs earlier this week, Ms Patel said this would be contingent on France preventing three out of four Channel crossings by the end of September.
The French interior ministry hit back commenting: “The terms of this funding were negotiated in detail with the British side and there was never any question of making payment conditional on quantified targets.
“Such an approach would reflect a serious loss of confidence in our co-operation.”
More to follow…