As the clock ticks down to Britain's scheduled exit on March 29, May is trying to persuade the European Union to change a deal that was agreed between London and Brussels late past year but overwhelmingly rejected by parliament in January.
Labour has said it will seek to force Mrs May into a decisive second Commons showdown on her Brexit deal by the end of the month.
Meanwhile, in a joint statement with Theresa May, European Commission President Jean-Claude Juncker said he had "underlined that the EU27 will not reopen the Withdrawal Agreement" but added that the political declaration on the future relationship could be "more ambitious in terms of content and speed".
Luxembourg Prime Minister Xavier Bettel says the U.K.is a "disunited kingdom" where pro-Brexit politicians lack ideas and courage. In January, a third of Conservative lawmakers joined opposition parties in rejecting her deal, delivering May the worst Commons defeat ever suffered by a sitting prime minister and throwing the Brexit process into deeper turmoil.
His remarks to reporters came after talks with EU Brexit negotiator Michel Barnier.
Liam Fox's trade department has secretly admitted to some of Britain's biggest businesses that the 40 free trade deals the minister promised to have ready by Brexit day are "unlikely" to materialise.
Angus MacNeil, chairman of the Commons global trade committee, said the admission by officials about new trade deals not being ready in time for Brexit was "what has been suspected for some time".
Theresa May is planning on revealing her latest Brexit plan a day early in order to give MPs "time to digest the content". "That isn't right in terms of the respect for parliament", said Starmer.
With only 46 days to go, the head of the International Monetary Fund, Christine Lagarde, warned that the British exit from the European Union means it "will never be as good as it is now" for the country's economy.
According to papers to be discussed by the Cabinet's Brexit sub-committee on Monday, the European Commission has told members states to "refrain from bilateral agreements or discussions with the United Kingdom, which would undermine EU unity".
Justice Minister Rory Stewart told the BBC on Monday that differences between the two aren't as great as some suggest, but the government can't accept a customs union that would prevent Britain from negotiating trade deals with other countries.
Labour leader Jeremy Corbyn has written to the Prime Minister setting out five demands that would have to be met for his MPs to support a deal, including a permanent customs union and close alignment with the single market. They suggested it was now time for Jeremy Corbyn to do so.