Because of the Fixed Terms Parliament Act, the prime minister cannot simply call an election.

Under the terms of the act, Mr Johnson would require the backing of two-thirds of the UK's 650 MPs to trigger an early poll this autumn.

Alternatively an election could also take place, if MPs were to approve a no confidence motion in the government.

Image source, Reuters

The prime minister is considering seeking an early general election if MPs wanting to block a no-deal Brexit defeat the government this week.

Those opposed to a no deal could chose to hold an emergency debate in order to pass legislation blocking a no deal. They could also pass a vote of no confidence in the government.

Read more about ways MPs could stop a no-deal Brexit here.

There is not scheduled to be another general election until 2022.

Elections traditionally take place on a Thursday and, in law, 25 days have to elapse between the current Parliament being dissolved and the polls opening.

So, if MPs vote for an election this week and Parliament is dissolved on Thursday, the earliest date that could come into play is 10 October.

Requesting extension:

• If, by 19 October the government hasn’t had a deal approved or a no deal approved, the PM must ask for an extension to 31 January 2020
• If either a deal or a no-deal motion is passed between 19 October and 31 October, the PM can change or withdraw the request

EU alternative date:

• If the EU proposes a different extension date, the PM must agree to the proposal within 2 days…
• …Unless MPs reject the proposal in a vote
• PM can still agree to a different extension

Publishing reports:

• If the extension is agreed, the government has to publish a report on negotiation progress by 30 November
• This has to be approved by MPs within 5 days
• If it is amended or rejected, another report has to be published by 10 Jan 2020
• There will be another report at least every 28 calendar days starting on 7 Feb 2020 until an agreement with the EU is reached “or until otherwise indicated by a resolution of the Commons”

There are less than two months before the UK is due to leave the EU. So what might happen next? Things are changing by the minute, but BBC senior elections and political analyst Peter Barnes sets out some scenarios.

Vicki Young
Chief Political Correspondent

We don't know what he is going to say. It could be he is just laying out how he sees things as we go into this momentous week.

It may well be, he is saying to his own MPs: "You can back me on Brexit, I am trying to get a deal, don't undermine me by blocking a no deal.

"And if you do do that you are potentially giving the keys of Downing Street to Jeremy Corbyn."

Image source, Reuters

The former Downing Street spin doctor Alastair Campbell, who worked under Tony Blair, says it is "highly unlikely that a general election will solve anything".

He argues Brexit "will only get legitimacy" through another referendum.

"That is the right and democratic thing to do.

"All of the maneuverings of the government are about running away from their deep fear that the country would now reject Brexit.

"Any Brexit. But especially the no deal Brexit the government knows will damage lives and livelihoods."

The PM's pledge has united a group of MPs across party lines who believe that leaving without a deal would be damaging.

They are expected to put forward legislation on Tuesday to stop no deal under "SO24" or Standing Order 24 - the rule allowing MPs to ask for a debate on a "specific and important matter that should have urgent consideration".

Among the Tory rebels are ex-Justice Secretary David Gauke and ex-Chancellor Philip Hammond.

The prime minister has said the UK must leave the EU on 31 October, with or without a deal, prompting a number of MPs to unite across party lines to try to prevent the UK leaving without an agreement.

Labour MP Hilary Benn has revealed on Twitter, external details of a bill which opponents of a no-deal Brexit will attempt to pass.

He said that it sets a deadline of 19 October for the prime minister to agree a new deal with the EU or get Parliament to explicitly agree to leaving without a deal.

If that isn't achieved, it would require Boris Johnson to request an extension of Article 50 until 31 January 2020.

He says: "The Bill has cross-party support from MPs who believe that the consequences of no deal for the economy and the country would be highly damaging. No deal is not in the national interest."

Earlier today Labour leader Jeremy Corbyn said the UK "needs" a general election, despite warnings from Tony Blair that such a vote would be "an elephant trap" for Labour.

The ex-PM warned that Boris Johnson could win such a vote as "some may fear a Corbyn premiership more" than a no-deal Brexit.

But Mr Corbyn said a vote "would give the people a choice between two very different directions for the country".

"An election is the democratic way forward," he added.

Read more here.

Boris Johnson is expected to address the nation outside No 10 at 18:00 BST, setting out the choices facing MPs.

The cabinet are gathered inside Number 10. BBC political editor Laura Kuenssberg says security is tight.