While a parliamentary technicality, it led to a vote in which another tranche of Tories rebelled, 17 this time, and which means that rather than having 21 days to come up with a Plan B if her deal is rejected next Tuesday - as is widely expected - the Prime Minister will have just three.
Speaker John Bercow was subject to jeering in the chamber on Wednesday morning when he announced that parliament would be allowed to vote on the amendment, as parliamentary rules usually only allow a government minister to make changes to motions of this type.
"If that is a choice then I say let's go on WTO rules".
When the Prime Minister chose to delay the meaningful vote on her Brexit deal by a month because she was facing a historic defeat, with more than 100 Tories ready to vote it down, she pledged to gain legally binding assurances from the EU.
A cross-party group of MPs sought Tuesday to amend a finance law to limit the government's tax-raising powers in the event of "no deal", as a way of forcing it to stop such a scenario.
May postponed an initial vote last month in the face of opposition from all sides of the House of Commons, but has now set it for next Tuesday evening after 1900 GMT, following five days of debate which start on Wednesday.
"The appropriate time to table a motion of no confidence in the government is when the government loses its key legislation and no longer commands a majority in the House of Commons".
With less than three months before Britain is due to leave the European Union, parliament has begun a five-day battle over May's Brexit plan, set to culminate in a vote by MPs next Tuesday.
Treasury minister Robert Jenrick said it is the " simple truth" that the United Kingdom would leave the European Union on 29 March, noting that planning for a no-deal scenario was "prudent preparation to provide our taxpayers with the certainty they deserve", adding that the only effect of the amendment would be to make the United Kingdom "somewhat less prepared".
"It is a warning to the Government not to drift into No Deal at the end of March by accident or through brinkmanship", she said.
A majority of members of Parliament oppose a no-deal Brexit, but it remains the default option if May's deal is rejected.
He later yelled "ridiculous" and "that is utter sophistry", but Mr Bercow defended his decision.
"I am absolutely hopping mad", she said.
Angry Tory Brexiteers questioned the Speaker's impartiality over Brexit in heated exchanges.
Commons Leader Andrea Leadsom noted there were "some concerns" about Mr Bercow's decision and asked him to confirm that he acted with "full advice" from the Commons clerk and other parliamentary advisers.
The government needs 318 votes to get a deal through the 650-seat House of Commons, as seven members of Irish nationalist party Sinn Fein do not sit, four Speakers and deputy Speakers do not vote, and the four tellers are not counted.
She set out further clarifications she hopes will win over her own Conservative MPs and Northern Ireland´s Democratic Unionist Party (DUP), which props up her government in parliament.