Brussels must be “open to a long extension” of Britain’s withdrawal negotiations if the UK decides to adopt a new approach to Brexit, the president of the European Council has said.

Donald Tusk warned that any attempt to stop Britain taking part in upcoming European Parliament elections – which the EU regards as necessary if the departure date is to be delayed beyond May 22 – would be a “betrayal” of those who marched in London last weekend or signed the petition to halt Brexit.

But Brexit Party MEP Nigel Farage warned the Parliament that if the UK was forced to take part in the May 23-26 polls, it would return large numbers of Leave MEPs – including him – to Brussels.

READ MORE: Brexit latest: Live updates as MPs prepare for indicative votes

Meanwhile, the European Parliament’s Brexit co-ordinator Guy Verhofstadt said that the EU would be ready to negotiate a “deeper and broader” future relationship with the UK if this was the outcome favoured by MPs taking part in indicative votes on withdrawal options in Westminster.

He said that Saturday’s march, which organisers said was attended by more than one million people, and the petition of more than 5.8 million signatures to revoke Article 50, represented the “seed” of an eventual UK return to the EU.

European Council president Donald Tusk addressed the European Parliament in Strasbourg (European Commission Audiovisual Services/PA Images)

Speaking to MEPs in Strasbourg, Mr Tusk urged the European Parliament to “be open to a long extension, if the UK wishes to rethink its strategy”.

He criticised MEPs who told him that possible UK participation in European elections would be “harmful or inconvenient”.

“Let me be clear, such thinking is unacceptable,” said Mr Tusk.

“You cannot betray the six million people who signed the petition to revoke Article 50, the one million people who marched for a People’s Vote, or the increasing majority of people who want to remain in the European Union.

READ MORE: EU leaders push back Brexit date as petition hits 2m

“They may feel that they are not sufficiently represented by their UK Parliament, but they must feel that they are represented by you in this chamber because they are Europeans.”

The seed for Britain’s return to the European Union was planted last weekend by marching in London & nearly six million signing the article 50 petition.We need a deep EU-UK relationship that one day, I’m pretty sure, will lead Britain back into the family of European nations. pic.twitter.com/Tsvc8Udrok — Guy Verhofstadt (@guyverhofstadt) March 27, 2019

But former Ukip leader Mr Farage told MEPs: “The one thing that is inevitable is that we are headed for an Article 50 extension. You should ask yourselves: Do you really want that?

“Do you really want Brexit to utterly dominate the next couple of years of your business to the exclusion of your many other ambitions?

“Do you really want the UK to contest the European elections, to send back a very large number of Leave MEPs, just at a time when you are fighting populism – as you see it – across the continent?”

And, to cries of “No” from some MEPs and “Yes” from others, he asked: “Do you really want me back in this place?”

EU chief Brexit negotiator Michel Barnier replied: “Mr Farage, no-one in Brussels is trying to steal Brexit from you, no-one is trying to undo the vote of the British people.

READ MORE: SNP MEP Alyn Smith calls on EU to 'leave a light on for Scotland'

“It is not Brussels that decided that the UK would leave the EU. You were the ones who made that choice and you are the ones who have to take your responsibility and face up to the consequences of that decision. No-one else.”

Appeal to EP: You should be open to a long extension, if the UK wishes to rethink its strategy. 6 million people signed the petition, 1 million marched. They may not feel sufficiently represented by UK Parliament but they must feel represented by you. Because they are Europeans. — Donald Tusk (@eucopresident) March 27, 2019

Mr Verhofstadt said that he hoped the indicative votes procedure taking place in London marked “the start of a cross-party co-operation to break the deadlock”, following repeated votes against Theresa May’s deal.

“A majority to break the Brexit deadlock will never have the support of the so-called hard Brexiters,” said the former Belgian prime minister. “The only thing that counts for them is to seize power inside the Tory Party. This is not in the national interest of the country.”

He said MEPs were “open” to revising the Political Declaration on the future framework for EU/UK trade and security relations, agreed last November alongside Mrs May’s Withdrawal Agreement.

READ MORE: European Union’s Guy Verhofstadt hails ‘Brexit revolt’ in UK

It would be possible to change it to make it a “more binding agreement between the EU and the UK” envisaging “a deeper and broader relationship in the interest of both Britain and the European Union”, he said.

Mr Verhofstadt added: “The seed for Britain’s return to the European Union was planted last weekend by marching in London and nearly six million signing the Article 50 petition.

An estimated one million people marched in London (Yui Mok/PA)

“We need a deep EU-UK relationship that one day, I’m pretty sure, will lead Britain back into the family of European nations.”

DUP MEP Diane Dodds told the Parliament that endangering the constitutional and economic integrity of the United Kingdom was not a price the Northern Irish party is willing to pay to ratify Mrs May’s agreement.

Mrs Dodds said the proposed Withdrawal Agreement “would see Northern Ireland leaving on different terms to the rest of the United Kingdom under the backstop”.

READ MORE: Brexit: MPs consider range of options for indicative votes

And she added: “The DUP wants to leave the European Union in an orderly fashion but this deal will endanger the constitutional and economic integrity of the UK. It is not a price that we as Unionists are willing to pay.”

European Commission President Jean-Claude Juncker voiced uncertainty about how the Brexit process will turn out.

“If I were to compare Great Britain to a sphinx, the sphinx would be an open book by comparison,” he told MEPs.

“And let’s see how that book speaks over the next week or so.”