Banners at a pro-Brexit protest in London on 29 March.
On April 1, MPs will use the same system to vote on a narrower set of options. In an effort to win support for her deal, May said she wouldn't lead those talks.
Labour leader Jeremy Corbyn refused to say whether his party would offer an option to remain in the European Union during these votes, but said the obvious choice was "a good economic relationship with Europe".
Britain's electorate voted by 52 per cent in favour of leaving the European Union back in 2016.
The other would likely keep Britain from setting off to strike its own independent trade deals.
"Since it took two and three-quarter years for the Government to get what it had negotiated defeated three times, it's a little bit harsh on Parliament, when it started the process last Wednesday, for not having immediately solved the problem in 24 hours", Mr Benn said.
That leaves the prospect of calling a general election in hopes of securing a stable majority government in Britain.
Whether Mrs May will still be PM over the next few weeks is also up in the air. Per the earlier agreement, the United Kingdom will be expected to indicate a way forward for leaders' consideration.
Cabinet member and chairman of the Conservative Party, Brandon Lewis, said he was aware of the letter, but had not seen the final text or the signatures.
Meghan to shun London hospital famed for royal births
The letter from the royal couple said, "My wife and I send our warmest wishes to you on the occasion of your 99th birthday on Friday".
Whilst the government is talking to the European Union about the terms of another extension to Brexit, the prime minister received a letter late Friday signed by 170 Conservatives MPs - a majority, including at least 10 Cabinet members - urging her to stick with the April departure date. Still, EU Council President Donald Tusk has urged the bloc to give Britain a Brexit extension if it plans to change course.
Many Conservative MPs are opposed to an election - particularly with May at the top of the ticket, and with the party trailing Labour by five points in a Deltapoll on Sunday - and could vote against holding one.
Agreeing to seek a customs union, if demanded by MPs, could therefore trigger a mass ministerial walkout.
British Labour leader Jeremy Corbyn on 30 March.
He said: "I'm convinced at that after spending a lot of time meeting with and talking to officials in Europe".
In order for this to happen, there must be a general election, a second referendum or another round of indicative votes resulting in a consensus in the Commons.
Other options on the table include leaving the European Union without a deal on April 12, the new Brexit deadline set by the European Union, and a confirmatory referendum on May's divorce bill. May has a poor track record when it comes to taking such political gambles, as Conservative MPs will recall that in 2017, she called a snap election in the hope of increasing her small majority, but instead she disastrously lost her majority entirely - a reality that has haunted her government in trying to pass Brexit policies in the House of Commons. Slack said the prime minister "believes there is a majority in the House for leaving in an orderly way with a deal", and her agreement was the best on offer. Time is of the essence, we have got Brexit to deliver.
But Downing Street later said this was not an "inevitability".
There are eight options on the table, but what isn't clear is how many Commons speaker John Bercow will put forward for votes - scheduled for 8pm tonight.
Another defeat would spell the end of the agreement, unless she attached the withdrawal agreement to a plan for a softer future relationship.
It split opinion last week in the Commons, with 268 votes for it and 295 against.