Britain has passed a long-elusive milestone on the road to Brexit, with the House of Commons approving a bill authorising the country's departure from the European Union at the end of the month.

Key points: Brexit Secretary Stephen Barclay said he welcomed the "constructive scrutiny" but hoped the Upper House would not try to delay the bill

Brexit Secretary Stephen Barclay said he welcomed the "constructive scrutiny" but hoped the Upper House would not try to delay the bill Ian Blackford of the Scottish National Party said Scotland now needed to be granted a referendum on independence

Ian Blackford of the Scottish National Party said Scotland now needed to be granted a referendum on independence Chief EU Brexit negotiator Michel Barnier said a full, free-trade agreement by the end of the year was unrealistic

Politicians voted 330-231 to pass the Withdrawal Agreement Bill on Thursday (local time), which sets the terms of Britain's departure from the 28-nation bloc.

The comfortable majority won by Prime Minister Boris Johnson's Conservatives in an election last month secured the bill's passage despite the opposition of smaller parties.

The bill was approved after three days of debate that brought none of the frayed tempers, late-night sessions and knife-edge votes that marked previous rounds of Brexit wrangling over the past year.

After passing through Parliament's unelected House of Lords — which can delay but not overturn the result in the Commons — the bill should become law in time for the UK to leave the EU on the scheduled date of January 31 and become the first nation ever to quit the bloc.

Brexit Secretary Stephen Barclay said he welcomed the "constructive scrutiny" of the Lords but hoped the Upper House would not try to delay the bill.

"I have no doubt that their lordships will have heard the resounding message from the British people on the December 12," he said.

Thursday's vote was a major victory for Mr Johnson, who has made delivering Brexit the key aim of his premiership.

MPs voted 330-231 to pass the Brexit Withdrawal Agreement Bill. ( Reuters: Tolga Akmen )

Britain voted narrowly to leave the EU in a 2016 referendum. But before the December 12 election, MPs repeatedly defeated attempts by both Mr Johnson and predecessor Theresa May to secure backing for their Brexit blueprints.

Conservative politicians cheered when the result of the vote was announced.

But Ian Blackford of the Scottish National Party said Scotland — which voted to remain in the EU in 2016 — must now be granted a referendum on independence.

Mr Johnson has said that would not happen.

"This is a constitutional crisis, because we will not and we cannot accept what is being done to us," Mr Blackford said.

'Difficult step yet to come'

Despite Mr Johnson's repeated promise to "get Brexit done" on January 31, the departure will only mark the start of the first stage of the country's EU exit.

Britain and the EU will then launch into negotiations on their future relationship, racing to strike new relationships for trade, security and a host of other areas by the end of 2020.

The EU's Michel Barnier said striking a full, free-trade agreement by the end-of-year deadline was unrealistic. ( AP: Jean-Francois Badias )

"Leaving the EU doesn't mean that we will have got Brexit done," Paul Blomfield, a Brexit spokesman for the main opposition Labour Party, said.

"We'll have completed the first step, departure, but the difficult stage is yet to come."

Top officials in the bloc are already saying sealing a new deal will be tough.

Michel Barnier, the EU's chief Brexit negotiator, said on Thursday Britain's goal of striking a full free trade agreement by the end-of-year deadline that Mr Johnson insists on was unrealistic.

"We cannot expect to agree on every aspect of this new partnership," Mr Barnier said, adding, "We are ready to do our best in the 11 months".

International trade agreements typically take years to complete, but Mr Johnson has ruled out extending a post-Brexit transition period agreed by the two sides beyond the end of 2020, although the EU has offered to prolong it until 2022.

That has set off alarm bells among UK businesses, which fear Britain could face a "no-deal" Brexit at the start of 2021.

Economists say that would disrupt trade with the EU — Britain's biggest trading partner — and plunge the UK into recession.

Britain and the EU will have to strike deals on everything from trade in goods and services to fishing, aviation, medicines and security.

The EU insists there is no way to deal with all these issues in less than a year.

British officials have suggested they could carve the negotiations up into chunks, sealing deals one sector at a time.

The two sides also have conflicting demands that are likely to complicate negotiations.

'Zero tariffs, zero quotas?'

Mr Johnson said the UK was seeking a wide-ranging free trade deal with the bloc, but did not want to agree to keep all EU rules and standards. It wants to be free to diverge in order to strike new trade deals around the world.

The EU said the UK would not get good access to its market unless it agreed to alignment.

Top officials in the bloc say sealing a new deal will be tough. ( Reuters: Peter Nicholls )

EU officials worry Britain plans to cut environmental and employment standards in order to position itself as a low-regulation, low-tax competitor to the bloc.

The bloc has stressed the need for a level playing field in the upcoming trade negotiations, meaning access to the EU market will be linked to UK commitments to standards in areas including workers' rights and the environment.

On Thursday, Mr Barnier added that if Britain wanted as much access as possible to the bloc's market, it would not have unfettered freedom to subsidise its industry.

EU nations are bound by strict state aid rules, enforced by the powerful European Commission, to make sure there is no unfair competition inside the EU's vast single market.

Mr Barnier said state aid rules in any future trade deal would be more stringent than with nations like Canada or Japan, simply because of the physical proximity of the departing EU nation.

"We will ask necessarily certain conditions on state aid policy in the UK," Mr Barnier said.

"If the UK wants an open link with us for the products — zero tariffs, zero quotas — we need to be careful about zero dumping at the same time," he told a conference in Stockholm.

AP