She said the Brexit withdrawal deal, painstakingly negotiated over more than 17 months, was a good deal and, crucially, it was the only one on the table.
"There is not a better deal available," she told MPs, adding that EU leaders have also stressed the point.
"We can back this deal, deliver on the vote of the referendum, and move on to building a brighter future of opportunity and prosperity."
Or, she said, MPs can reject the deal and "go back to square one," exposing Britain "to more division and more uncertainty, with all the risks that will entail." Scores of MPs stood to denounce the plan.
Jeremy Corbyn, leader of the opposition Labour Party, told Parliament that the "botched" Brexit deal was "the worst of all worlds - no say over future rules and no certainty for the future". He cited one estimate that the British economy would shrink by 3.9 per cent with this deal in place.
"Ploughing on is not stoic. It's an act of national self-harm."
Boris Johnson, a leading Brexiteer who resigned as May's Foreign Secretary in July, said that to call May's plan "unsatisfactory" would be an understatement.
"It's very hard to see how this deal can provide certainty to business or to anyone else."
May's half-in, half-out compromise divorce deal has angered "leavers" and "remainers" alike. Brexiteers want a clean break from the European Union, and pro-Europeans think the deal is worse than the status quo.
In an intervention that didn't do May any favours, US President Donald Trump said that her Brexit deal "sounds like a great deal for the EU". He also said that it could hinder a trade deal between the United States and Britain.
If Parliament votes down this version of Brexit, Britain faces the possibility of leaving the EU in March with no deal in place, a "doomsday scenario" that could result in grounded airplanes, medicine shortages and economic havoc. Most political analysts predict that such a fate would be avoided. If May's deal is struck down, and if market chaos ensues, MPs might be willing to vote for it in a second go-round.
Mujtaba Rahman, an analyst with Eurasia Group, said what happens next would depend on the size of May's defeat. He predicted that if she loses the vote by a small margin, she will seek changes from the EUand aim for a second parliamentary vote, likely in January.
But a "heavy defeat" would increase the odds of other outcomes. Britain might work with the EU on a "managed no-deal" scenario or get European approval to extend the Brexit deadline beyond March. Some MPs are discussing a "Norway Plus" option that would see Britain join the European Free Trade Association. A second referendum in Britain or a general election would also be possible.
Tonge said that if the proposal doesn't get the support of Parliament, May's credibility would be in tatters because she is "so wedded to the deal". But May has proven time and again that she has staying power. She survived an attempted coup last week by Eurosceptics who failed, at least for the moment, to get enough MPs to launch a vote of confidence against her.
