The UK could remain subject to the European Court of Justice (ECJ) for years as part of a transitional deal with the EU, Europe’s top Brexit negotiator has warned.

A transitional deal - favoured by Theresa May - would follow the invocation of Article 50 until the withdrawal process has been completed, but could be in place a long time after 2019.

But Guy Verhofstadt, former Prime Minister of Belgium, said to the Guardian: “The starting point from the European side” would be to keep the UK part of the ECJ, as part of any transitional deal.

If true, this position would be extremely frustrating to the Prime Minister, who has been known for her opposition to the European Courts since her days at the Home Office.

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It would also be met with dismay from the Eurosceptic wing of the Conservative party and other Brexiteers, who are likely to view it as the EU continuing to prevent the UK from controlling its own affairs.

By the Prime Minister’s timetable, Article 50 will be triggered by the end of March. A Great Repeal Bill will also be passed, which is intended to end the primacy of EU laws in Britain.

But the UK will not have full control over its legal system if it remains part of the ECJ.

Mr Verhofstadt also insisted the UK would not be allowed to negotiate its exit from the EU at the same time as settling its future trade relations with the bloc.

"That is not possible. Technically (it is not possible) in the time we have, 14 or 15 months, let's be honest," he told the newspaper.

The negotiator said claims that any negative Brexit impacts on the City of London would hit the EU harder than the UK were part of a "psychological war".

How Brexit affected Britain's favourite foods from Weetabix to Marmite Show all 8 1 /8 How Brexit affected Britain's favourite foods from Weetabix to Marmite How Brexit affected Britain's favourite foods from Weetabix to Marmite Weetabix Chief executive of Weetabix Giles Turrell has warned that the price of one of the nation’s favourite breakfast are likely to go up this year by low-single digits in percentage terms. Reuters How Brexit affected Britain's favourite foods from Weetabix to Marmite Nescafé The cost of a 100g jar of Nescafé Original at Sainsbury’s has gone up 40p from £2.75 to £3.15 – a 14 per cent rise—since the Brexit vote. PA How Brexit affected Britain's favourite foods from Weetabix to Marmite Freddo When contacted by The Independent this month, a Mondelez spokesperson declined to discuss specific brands but confirmed that there would be "selective" price increases across its range despite the American multi-national confectionery giant reporting profits of $548m (£450m) in its last three-month financial period. Mondelez, which bought Cadbury in 2010, said rising commodity costs combined with the slump in the value of the pound had made its products more expensive to make. Cadbury How Brexit affected Britain's favourite foods from Weetabix to Marmite Mr Kipling cakes Premier Foods, the maker of Mr Kipling and Bisto gravy, said that it was considering price rises on a case-by-case basis Reuters How Brexit affected Britain's favourite foods from Weetabix to Marmite Walkers Crisps Walkers, owned by US giant PepsiCo, said "the weakened value of the pound" is affecting the import cost of some of its materials. A Walkers spokesman told the Press Association that a 32g standard bag was set to increase from 50p to 55p, and the larger grab bag from 75p to 80p. Getty How Brexit affected Britain's favourite foods from Weetabix to Marmite Marmite Tesco removed Marmite and other Unilever household brand from its website last October, after the manufacturer tried to raise its prices by about 10 per cent owing to sterling’s slump. Tesco and Unilever resolved their argument, but the price of Marmite has increased in UK supermarkets with the grocer reporting a 250g jar of Marmite will now cost Morrisons’ customers £2.64 - an increase of 12.5 per cent. Rex How Brexit affected Britain's favourite foods from Weetabix to Marmite Toblerone Toblerone came under fire in November after it increased the space between the distinctive triangles of its bars. Mondelez International, the company which makes the product, said the change was made due to price rises in recent months. Pixabay How Brexit affected Britain's favourite foods from Weetabix to Marmite Maltesers Maltesers, billed as the “lighter way to enjoy chocolate”, have also shrunk in size. Mars, which owns the brand, has reduced its pouch weight by 15 per cent. Mars said rising costs mean it had to make the unenviable decision between increasing its prices or reducing the weight of its Malteser packs. iStockphoto

"I don't believe in this catastrophic scenario: without the City of London the whole thing collapses. I think this is completely, well, (much) of what you hear today is the building up of the muscles in the negotiation. Fine, it is good. But that is more creating the environment of psychological war."

Mr Verhofstadt also expressed concern about the effects of Brexit on the Northern Irish peace process.

"Everybody is anxious and saying we don't want to return to the past,” he said. “If you take Brexit as it is then, yes, you create a hard border again. Nobody thinks that is a good solution.

"How to have a Brexit and no hard border? That is the question and it is not only through some technical innovations, cameras and sensors ([on a customs border). That's a real concern."