A THREAT to stop British planes from flying over Ireland led to its leader being branded “mad” last night.

In a Brexit strop, Irish PM Leo Varadkar said it would be a response to Theresa May cutting Irish fishermen’s access to our waters.

8 Leo Varadkar has issued an extraordinary warning to Theresa May Credit: PA:Press Association

The extreme action would be a tit for tat punishment for the Theresa May cutting Irish fishermen’s access to our waters.But as a furious row erupted between London and Dublin, UK diplomatic sources pointed out that Mr Varadkar has got his facts wrong - and dubbed his claim “pretty foolish”.

The row threatened to overshadow Theresa May’s 24 hour visit to Northern Ireland, which began last night, to sell her Chequers plan for a softer Brexit.

Highlighting the importance of keeping the Irish border open, the PM will step up her challenge on hard Brexiteers today (FRI) to stop dealing with the EU exit in high theory and “make a success of it in practice”.

Mr Varadkar lashed out while taking his Cabinet on an away day to step up planning for no deal. He said: “The situation at the moment is that the UK is part of the single European sky.

8 Hundreds of thousands of flights from Ireland currently cross UK airspace every year Credit: Alamy

“If they leave the EU they are not, and that does mean if there was a no-deal hard Brexit next March, the planes would not fly and Britain would be an island in many ways. If they want their planes to fly over our skies, they would need to take that into account.”

The Ireland boss insisted: “You can’t have your cake and eat it. You can’t take back your waters and then expect to take back other people’s sky”.

A DExEU spokesman said: “We do not want or expect a no deal scenario. Our White Paper sets out both our desire to agree a mechanism for annual negotiations on access to waters and fishing opportunities, as well as reciprocal liberalised aviation access with the EU through an Air Transport Agreement.”

But a senior UK government source branded the comments “mad”.

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8 The Government has said it will be assuming control of fishing quotas after Brexit Credit: Getty - Pool

British officials pointed out that freedoms of the air are not guaranteed by the EU, but a multilateral international treaty – the International Air Services Transit Agreement - signed from 1944 onwards, by 133 countries.

The only way Ireland can block British planes flying over it would be to pull out of the treaty, which it has been a signatory to since 1957. Others said Britain could retaliate by blocking Irish planes from flying to mainland Europe over the UK, doubling the length of their journies.

The Director of the Open Europe think tank Henry Newman said Mr Varadkar is “in danger of losing the plot”.

Alex Macheras, Aviation Analyst, said “It’s a ridiculous statement; and the reason why ICAO’s International Air Services Transit Agreement exists - of which Ireland are signatory - which ensures countries cannot simply ‘impose a ban’ on their airspace, and the treaty specifically states that countries are forbidden from discriminating against the nationality of an aircraft.

The Sun Says IRISH Prime Minister Leo Varadkar is a bigmouth and a fool, but his latest bone-headed outburst has done us all a favour. It has exposed beyond doubt both his crass naivety and the cynical deceit of the EU masters he cravenly obeys. Imagine the cataclysmic self-harm of Ireland preventing planes from its biggest trading partner flying over it. And the impossibility of its own airlines avoiding UK airspace. It won’t happen. Varadkar must believe that these grenades he brainlessly lobs into the Brexit negotiations help Brussels. He is not bright enough to realise how absurd and unstatesmanlike he looks. But he let something else very revealing slip too. Varadkar reportedly said that EU leaders have assured him no hard border or customs checks will ever be needed between Ireland and Northern Ireland. That’s under ANY scenario, including a No Deal. Since we will never build a hard border either, why is “avoiding” one still being discussed, let alone a huge sticking point? Answer: It is a ruse, confected by a Brussels desperate to keep us in its grip.

8 His extreme action to ban UK planes from Ireland's airspace would be a tit-for-tat punishment were Theresa May to cut Irish fishermen’s access to our waters Credit: AFP or licensors

“Ireland has very little airspace; and if a ban were to take place Ireland would lose all flights operated to/from the country flown by aircraft with G- registrations, Great Britain. This is without mentioning that much of Ireland’s airspace management, and air traffic control, is actually based in the UK.”

Brexiteer MP Jacob Rees-Mogg told The Sun: "Air traffic control continued between Russia and the Ukraine after Russia invaded the Crimea so this idea is just silly.

"On the other hand most flights from the EU to America pass through our air traffic control so this rather lightweight Irish gentleman is proposing an absurd act of a masochistic nature. His words are those of an airhead."

Andrew Rossindell MP said: "Who will this benefit? Mr Varadka needs to think very long and hard before he makes statements like this. It's incredibly damaging to British-Irish relations and very short-sighted. If he thinks this would benefit Ireland, he needs to think again. In reality it would just be catastrophic for everyone.

8 The clash has threatened to overshadow Theresa May’s 24-hour visit to Northern Ireland to sell her Chequers plan for a softer Brexit Credit: AP:Associated Press

"If his ideological obsession with the EU means Ireland can't have sensible relations with its closest neighbour and friend, it's a tragedy and totally outrageous. He should stop taking his orders from Brussels and start thinking about the greater good of the people of Ireland and the whole of the British Isles.

"Ireland will always be tied to Britain, no matter what happens, and if it harms us it will harm them. He needs to change his attitude very urgently."

Last night RAF legend John Nichol said: “Regardless of your views on Brexit, the notion that you can have a one way system is farcical and non sensical. It’s a ludicrous notion you can have a ban on civilian aircraft coming to one area but when you desperately need the Brits’ help you change your mind.

“That would be strange and incomprehensible. “You really cannot have it both ways. We have spent years building up warm relations with Ireland. They should not be a republic of Brussels and think of their own long-term national interest. The EU will use them and spit them out, but we are genuinely Ireland's best friend.

8 A record 1.13m flights operated in Irish airspace last year, according to the Irish Aviation Authority Credit: PA:Press Association

"Mr Varadka is playing a very dangerous game politically in trying to prevent Brexit."

In a second rumpus, Mr Varadkar was also accused of hypocrisy after he claimed the EU had promised him that no physical checks will need to be erected on the Irish border even if the UK crashes out with no deal.

Under the “doomsday scenario”, the Taoiseach said the “commitments of others” would then kick in to prevent a hard border. But Brexiteers leaped on his declaration to insist that has been Britian’s argument all along.

Tory MEP Dan Hannan said: “Ireland confirms it won’t put up physical border checks in any circumstances, including a no-deal scenario. The UK has said the same from the outset. So what the hell are we arguing about?”

8 Dublin to London is the second-busiest route in world as almost 4.5million people flew between the Irish and British capitals last year Credit: PA:Press Association

Critics also said Mr Varadkar’s declaration was in contravention of the World Trade Organisation’s rules, which insist on borders between different customs regime being policed. Mrs May visited the Irish border town of Belleek in Fermanagh last night to meet local businesses and hear their concerns.

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This morning, the PM will deliver a speech in Belfast to call on hard Brexiteers who have rejected her softer Brexit Chequers deal to start addressing the reality on the ground. And she will insist that is their “duty”, as the future of the UK is at stake if they don’t.

Mrs May will say: “Our job is not to deal with Brexit in theory, but to make a success of it in practice for all of our people. For all of us who care about our country, for all of us who want this Union of nations to thrive, that duty goes to the heart of what it means to be a United Kingdom and what it means to be a government.”

But former Brexit minister David Jones challenged the PM to claim the Irish border issue has been “weaponised” by the EU to get its way in the negotiations.