Theresa May has conceded that exit talks with the European Union could be tougher than previously admitted.

The Prime Minister told MPs that the negotiations could go on until almost the last possible moment.

And she warned that the details of the Brexit agreement might not be finalised until just weeks before the UK leaves the EU.

Her comments follow reports that the EU has privately told ministers it will push for agreement on the UK’s ‘divorce bill’ before doing a deal on British citizens living on the continent.

Mrs May is under pressure to guarantee that the three million EU citizens currently living in the UK can stay after Brexit.

She has insisted that she needs assurances on the rights of UK nationals in EU countries first.

But Brussels is pressing for the initial agreement in the talks to be over cash.

However, the UK will not have to sign up to a final sum immediately, just agree to 'broad principles' according to the BBC's Newsnight programme.

Experts estimate that the EU could demand as much as £50bn as the UK leaves.

European leaders argue that the UK‘s has ‘legacy’ responsibilities on everything from planned multi-billion-pound infrastructure projects to pension liabilities.

Last month a report by House of Lords committee concluded that the UK could legally walk away without paying a penny.

But peers also warned that Britain would suffer a high political price if it did.

In response, sources suggested that Mrs May could also call on Brussels to hand back £9bn of UK assets held by an EU bank.

The EU's chief negotiator Michel Barnier has called for a Brexit deal by October 2018 to get any agreement through the European Parliament before the two-year talks deadline.

At the time, Foreign Secretary Boris Johnson described that as “ample” time.

But Mrs May told MPs: “We have a timetable of up to two years for the negotiations, and it is possible that the details will not be finalised until close to the end of that period.”

No 10 said that the timetable for negotiations still had to be agreed with the EU.

Later, however, Mrs May insisted that the historic parliamentary vote on Brexit this week had strengthened her hand in negotiations.

She said: "I think we can be reassured that the vote that took place in this House and the House of Lords last night, and the passing of the European Union (Notification of Withdrawal) Bill into royal assent, will give a very clear message to everybody in Europe that we mean business."

Meanwhile, the immigration minister has suggested that different immigration rules for EU nationals could apply to different sectors of the economy after Brexit.

Robert Goodwill said he did not want to speculate on whether Britain could bring in a visa-based, a points-based, or another immigration system for EU citizens after leaving the bloc.

The Conservative Government has repeatedly rejected the Scottish Government's call for a separate immigration system for Scotland post-Brexit.

London Mayor Sadiq Khan has told MPs that financial sector businesses could start making plans to move operations out of the UK as soon as Theresa May triggers EU withdrawal talks.

Earlier this week Mrs May backed away from a widely expected timetable to start exit talks.

No 10 said that she would not trigger Article 50 of the Lisbon Treaty and start the two-year process of leaving the EU this week.

Instead, Mrs May will give notice of the UK's intention to quit the organisations by the final week of March.

Meanwhile, leaked Whitehall documents stating that at least seven new bills will be required in key areas affected by Brexit

Trevor Tayleur, associate professor at The University of Law, warned that could place greater pressure on the parliamentary timetable in the UK “if all the required legislation is to be in force by March 2019, the anticipated date of the UK’s withdrawal from the EU.”