A 36-year-old Bangladesh-origin British lawmaker has delayed giving birth to vote on the UK's landmark divorce deal with the EU on Tuesday.

Tulip Siddiq, a Labour lawmaker and niece of Bangladesh Prime Minister Sheikh Hasina, has been advised by doctors to have a Caesarean section, but agreed to push the procedure back to Thursday so she can vote on the Brexit deal on Tuesday in the House of Commons.

By her decision, the Opposition lawmaker from Hampstead and Kilburn, has reigniting the debate over proxy voting in Parliament, the BBC reported.

Siddiq told the Evening Standard that she had a difficult first pregnancy with her two-year-old daughter, and was originally due to give birth to her second child by elective caesarean section on February 4.

But after developing gestational diabetes, her doctors recommended she bring the date forward to the coming Monday or Tuesday.

She spoke to medical staff at the Royal Free Hospital in Hampstead, London, and they agreed to the delay.

Siddiq said: 'If my son enters the world even one day later than the doctors advised, but it's a world with a better chance of a strong relationship between Britain and Europe, then that's worth fighting for.'

British MPs are preparing to vote on Tuesday on whether to back embattled Prime Minister Theresa May's deal for leaving the European Union on March 29, 2019.

May has called for politicians to back her deal or risk 'letting the British people down'.

But with many of her own MPs expected to join Opposition parties to vote against the deal, it is widely expected to be defeated, British media reported.

Siddiq, the grand-daughter of Sheikh Mujibur Rahman who was the first president of Bangladesh, said she plans to go through the lobby of the House of Commons in a wheelchair. Hasina is her aunt.

Meanwhile, Speaker of the Commons John Bercow said that a proxy vote for Siddiq would be his 'preference', but it was not in his power to grant it. However, he was happy for her to be 'nodded through'.

This process normally means an MP's vote can be counted if they are anywhere on the parliamentary estate, rather than going through the voting lobby.

It has traditionally been used for unwell MPs, such as during the votes on the Maastricht Treaty in the 1990s, where some were driven onto the estate by ambulance to be 'nodded through' on crucial votes.

There is a system in Parliament for MPs who cannot make a vote called 'pairing' - where an MP on each side of an issue agree to not vote, so cancelling each other out.

However, Siddiq said she did not trust the system after Tory chairman Brandon Lewis was accused of breaking the system when he voted on a Trade Bill, despite being paired with Liberal Democrat Jo Swinson, who was on maternity leave. He later apologised for making an 'honest mistake'.

'If the pairing system is not honoured, there's nothing I can do, and it's going to be a very close vote,' said Siddiq. 'I've had no pressure at all from the whips to come and vote but this is the biggest vote of my lifetime. I've sat down with my husband Chris and he said to me this is my choice, but that he would support me,' she said.