The EU said Tuesday it believes it has done "all in our power" to ensure an orderly Brexit and that a new divorce deal now hangs on the approval of British MPs before any European ratification.

Expressing regret that Britain planned to leave the EU on October 31, European Commission chief Jean-Claude Juncker said: "At least we can look ourselves in the eyes and say that we have done all in our power to make sure that this departure is orderly."

He and European Council President Donald Tusk, addressing the European Parliament in Strasbourg, said developments in the British parliament in Westminster would be decisive on how Brexit proceeds.

"We need now to watch events in Westminster very closely," Juncker said. "It's not possible, not imaginable that this (European) parliament will ratify the agreement before Westminster will have ratified the agreement."

"It is obvious that the result of these consultations will very much depend on what the British parliament decides, or doesn't decide. We should be ready for every scenario," he said.

Lawmakers in Britain, meeting on a Saturday for the first time in 37 years, voted to demand that Prime Minister Boris Johnson delay Brexit while they further consider a divorce agreement agreed with the EU last week.

The government now wants parliament to approve an accompanying package of legislation that needs ratifying for Brexit to take effect.

The EU's Brexit negotiator Michel Barnier warned that even if a Brexit withdrawal agreement is ratified and comes into force, it would just be the start of even longer negotiations to decide the future EU-UK relationship.

"We will have to renegotiate for one year, two years, three years, maybe more in some areas, to rebuild all that will have been pulled apart by the desire of those backing Brexit," he said.

Barnier said rules would have to be established to address the risk of environmental, tax and labour "dumping", and in terms of cooperation for education exchanges, police, defence and foreign policy.