The UK will be handed a “very hefty bill” for leaving the European Union, Jean-Claude Juncker has warned.

The President of the European Commission said the EU would seek payment from Theresa May's Government to cover future spending Britain had committed to but would not fulfil as a member.

“The British should know this, they know this already, that it will not be at a discount or at zero cost. The British must respect commitments they were involved in making,” he told the Belgian parliament.

“So the bill will be, to put it a bit crudely, very hefty.”

Theresa May sits in House of Lords to hear Brexit debate

Mr Juncker said Britain would need years to negotiate any future trade deals with the EU after quitting the bloc.

“To agree on the future architecture of relations between the United Kingdom and the European Union, we will need years,“ he added.

Discussions are under way in Brussels on the size of the bill to be presented to the Prime Minister when she launches withdrawal talks, with unconfirmed reports suggesting the demand may be as high as €60bn (£51bn) over the next six years.

The sum would cover the UK's share of the cost of projects and programmes that it signed up to as part of the 28-nation bloc, as well as pensions for EU officials who served during its 45-year membership

Brussels’s position is that withdrawal arrangements – including the “exit bill” – must be agreed before moving on to potentially lengthy negotiations over future trade relations.

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But the Government insists the talks can run in parallel after Ms May kicks off negotiations under Article 50 of the EU treaties, which she has promised to do by the end of March.

The European Commission has told the UK there can be no membership of the European single market without freedom of movement for workers – a key issue for many people who voted for Brexit.

“Those who want to benefit from the advantages of the single market must respect the four fundamental freedoms, including the one which relates to the movement of workers,” Mr Juncker said, calling Brexit “a crisis which concerns us all”.

“We need to settle our affairs not with our hearts full of a feeling of hostility, but with the knowledge that the continent owes a lot to the UK,” he added.

“Without Churchill, we would not be here – we mustn’t forget that, but we mustn't be naive.

“Our British friends will need to understand that we want to continue to develop European integration.”

His comments came during the second day of the House of Lords debate on the European Union (Notification of Withdrawal) Bill.

The legislation, which would allow the Prime Minister to trigger the formal two-year Brexit process, has already cleared the Commons unamended with big majorities.

But the Government does not have a majority in the Lords and opposition peers are attempting to amend or block the legislation.