The European Parliament said on Wednesday that Brexit could be reversed, but if Britain pursued divorce from the EU it should first settle its bill and protect citizens' rights before a new trade deal with the bloc can be discussed.

In a draft resolution published on the day that Britain formally triggered the two-year process to leave the European Union, senior EU lawmakers said there could be transitional arrangements to smooth the UK's departure but they should not last more than three years.

Two London-based EU agencies, the European Banking Authority and the European Medicines Agency, must also move as soon as practicably possible, said the draft resolution.

The draft sets the European Parliament's red lines on the conduct of EU divorce talks with Britain before a free trade pact can be tackled. The Strasbourg-based assembly will have a final say on any deal struck with Britain.

In casting the Brexit process as reversible, EU lawmakers were seen to be offering a concession to strengthen the hand of Britons who voted to stay in the EU in last year's membership referendum, despite opposition from EU chief negotiator Michel Barnier to making any such reference.

Conservative British Prime Minister Theresa May declared on Wednesday there was no turning back from Brexit. Britons voted by a margin of 52 to 48 percent to leave the EU.

The EU parliament's resolution, to be voted on by the entire chamber next week, stressed that even if Britons changed their mind they would not get a better deal than what they had as members of the world's largest political and trade bloc.

Parliament president Antonio Tajani told a news conference that if Britain decided to reverse course, "it cannot do it alone (as) all member states need to decide whether it is possible (too)".

The EU parliament's Brexit team, led by Belgian ex-prime minister Guy Verhofstadt, accepted that in case of Brexit there should be transitional provisions to ease the split but lasting no more than three years and "strictly limited" in scope.

GUARANTEES

Negotiations on such arrangements could start only after substantial progress is made on a withdrawal accord that should include a bill for Brexit, guarantees for EU citizens living in Britain and British nationals residing in EU countries, and legal certainties for companies, the document said.

"Not reaching a deal on citizens' rights means not reaching a deal at all," Tajani said. Three million EU citizens live in the UK and about 1 million British nationals in EU countries.

The EU's parliament's draft resolution said London's Brexit bill should cover not just outstanding commitments to the bloc's budget but also "provision for off-balance sheet items, contingent liabilities and other financial costs that arise directly as a result of its withdrawal".

That raised the prospect of a tab even higher than the 60 billion euros ($64.5 billion) figure bandied about of late - an amount rejected by prominent pro-Brexit politicians in the UK.

Verhofstadt said it was possible to reach a withdrawal agreement and also one on the "general terms of the future relationship" within the two-year time frame of Brexit talks.

To that end, he said, Britain should recognise that the European Court of Justice will be the "competent authority for the interpretation and enforcement of the withdrawal agreement".

That stance may not go down well in Britain, where many see a quick end of ECJ jurisdiction as critical to restoring complete national sovereignty after four decades in the EU.