London (AFP) - A bill empowering Prime Minister Theresa May to start Brexit cleared the House of Commons Monday, as Scotland signalled its opposition to the divorce by calling for a new independence referendum.

MPs overturned amendments to the bill tabled earlier this month in the House of Lords, sending the legislation back to upper house for what could be final approval later this evening.

But May's spokesman sought to play down speculation that she would begin the process for leaving the EU as soon as Tuesday, when the bill is expected to become law following royal assent from Queen Elizabeth II.

"We have been clear that the prime minister will trigger Article 50 by the end of March," her spokesman said ahead of the vote, heavily emphasising the word "end".

The prospect of an imminent start to negotiations was enough, however, to push the nationalist devolved government in Scotland into calling for a fresh independence vote.

As the price for cutting immigration, May has said Britain will leave Europe's single market -- a move that the Scottish National Party (SNP) in power in Edinburgh has warned would be highly damaging to jobs and growth.

SNP First Minister Nicola Sturgeon has said since the June referendum vote for Brexit that Scotland, where a majority wanted to stay in the EU, sought a different future.

On Monday she made good on her threat, promising to give Scotland "a choice at the end of this process" by early 2019 -- before Britain leaves the EU.

The European Commission, however, quickly responded saying that Scotland would have to reapply to join the EU rather than inheriting Britain's membership.

- Countdown to Brexit -

May has the power to block the vote and said that another referendum, after Scots voted by 55 percent to reject independence in 2014, would only cause "uncertainty and division".

But Sturgeon's call pushes to centre stage one of the prime minister's biggest concerns about Brexit -- that it could lead to the break-up of the United Kingdom -- as she heads into complex negotiations.

The other 27 European leaders are prepared for Britain to trigger Article 50 of the EU's Lisbon Treaty, which begins a two-year countdown to Brexit, to be triggered as early as this week.

However, speculation is growing that it may now be delayed until after a March 25 summit in Rome to mark the EU's 60th birthday -- timing that would likely be welcomed in Brussels.

Once May has notified the EU of her decision by letter, the bloc will take just 48 hours to issue its first draft proposal for the negotiations, with a follow-up meeting planned on April 6.

While talks are not expected to begin for months, EU leaders are determined that the final terms do not encourage other member states to follow Britain and jump ship.

- 'Shame on you' -

May introduced the bill empowering her to trigger Article 50 after the Supreme Court ruled that only parliament could approve the start of Brexit.

The bill was held up when the Lords tabled amendments demanding guarantees for EU nationals' rights and a parliamentary vote on the final withdrawal deal.

Brexit minister David Davis had urged MPs to overturn the changes on Monday, saying: "We will not enter the negotiations with our hands tied."

The amendment demanding protections for more than three million Europeans living in Britain after Brexit was defeated by 335 votes to 287 -- prompting shouts of "shame on you" from protesters outside.

Around 150 people had gathered in support of efforts by the opposition Labour party to uphold the change, including Karin Templin, a 39-year-old architect who was born in the US but is now British.

"I'm appalled at the UK government, at this stupid ridiculous game that means they won't guarantee the rights of everybody who wants to stay in their home and in their jobs. I'm disgusted," she said.

May's government says it wants to guarantee Europeans' rights to stay in Britain, but cannot until EU leaders offer similar rights to British expatriates.

The other vote, which would have given parliament, rather than the prime minister, the right to decide whether to accept the final deal or walk away without agreement, was defeated by 331 votes to 286.