LONDON, Nov. 26 (Xinhua) -- British Prime Minister Theresa May faced her toughest test yet Monday as she started her task of "selling" her Brexit deal to disgruntled MPs in the House of Commons.

She told lawmakers that the choice was accepting the deal signed in Brussels Sunday or going back to "square one" with no certainty about the future.

Main opposition leader, Labour's Jeremy Corbyn said May's deal was a "botched" job, saying the negotiations between Britain and the European Union had failed.

He said the Brexit deal, endorsed Sunday by EU leaders in the special Brexit summit in Brussels, betrayed the Brexit people voted for in the 2016 national referendum, adding that the House of Commons would have to reject her deal.

MPs from other parties were also lining up to condemn the Brexit deal during the debate, which is seen as a curtain raiser to a debate in mid-December when MPs will vote on a legally binding Brexit deal.

May said the deal, which took EU leaders just 38 minutes to agree Sunday, was the right deal for Britain because it delivers on the democratic decision taken by British people in the referendum in June 2016.

In her statement, May said she recognized the concerns over the backstop, a measure included in the agreement to avoid a hard border between Northern Ireland and the Irish republic.

The backstop would only ever be used if no permanent trading deal was in place by Jan. 1, 2021. May said if it were to be implemented, it would only be temporary.

Telling MPs that both sides have had to make compromises, May added: "The backstop is essential. Without a backstop, there will be no agreement."

She added that under the deal, for the first time in 40 years Britain will be able to open up new trade deals around the world.

Insisting what was agreed in Brussels was the best deal available, May said no one knows what would happen if it does not pass.

With so many MPs from May's own Conservative Party as well as opposition MPs saying they will vote against her deal, political commentators are already predicting a possible defeat for the Brexit deal next month.

But May remained determined to see the deal through. "The British people want MPs to get on with a deal and allow the country to come together," she said.