British Prime Minister Theresa May has said her critics risk damaging democracy and warned the UK would be in "uncharted territory" if MPs do not back her Brexit plan.

As May seeks further guarantees from Brussels ahead of the parliamentary vote on the deal, expected on Jan 15, she warned pro-Brexit supporters they risked derailing the UK's departure from the European Union.

The prime minister said no alternative plan was able to respect the 2016 referendum result, protect jobs and provide certainty to citizens and businesses.

On BBC television on Sunday morning, May promised further measures on Northern Ireland and said "we are still working on" getting extra assurances from Brussels as part of her drive to secure support for the deal.

She sidestepped questions about what would happen if the deal kept being rejected, instead saying: "If the deal is not voted on, this vote that is coming up, then actually we are going to be in uncharted territory. I don't think anybody can say exactly what will happen in terms of the reaction we will see in Parliament."

In a Sunday newspaper article, she warned critics that they risked damaging the economy and trust in democracy if they opposed her plan.

"There are some in parliament who, despite voting in favour of holding the referendum, voting in favour of triggering Article 50 and standing on manifestos committed to delivering Brexit, now want to stop us leaving by holding another referendum.

"Others across the House of Commons are so focused on their particular vision of Brexit that they risk making a perfect ideal the enemy of a good deal.

"Both groups are motivated by what they think is best for the country, but both must realise the risks they are running with our democracy and the livelihoods of our constituents."

She again rejected calls for a second referendum, saying it would be disrespectful to people who voted for Brexit in 2016.

The prime minister said that the British"genius for pragmatism" had always found a way forward which commands consensus at"moments of profound challenge" such as this.

She said Labour's approach under Jeremy Corbyn had been based on a "cynical tissue of incoherence, designed to avoid difi cult decisions".

May was forced to postpone a vote on her plans in December after it became clear the deal would be rejected by MPs.

MPs will resume debate on the Brexit deal on Wednesday ahead of a vote the following week.

Meanwhile, an amendment to the fi nance bill could rob the treasury of its no-deal powers if ministers pressed ahead with a plan to crash out of Brussels without the support of MPs, and a second amendment would stop the treasury from raising income or corporation tax unless Parliament approved a Brexit deal.

The twin changes to the fi nance bill, being voted on by MPs on Tuesday, could throw the government into chaos if they are passed.