The vote of independent unionist Lady Harmon was crucial in the House of Commons on Wednesday night as legislation delaying Brexit passed by just one vote.
The bill, from Tory grandee Sir Oliver Letwin and former Labour minister Yvette Cooper, passed its third reading by the narrowest of margins, to the dismay of Brexiteers.
The European Union (Withdrawal) (No 5) Bill bill – part of a wider Commons attempt to prevent a no-deal Brexit – would force Theresa May to delay Brexit rather than risk a no-deal departure from the EU reinforces how the government no longer controls the Commons.
Just a few hours before the bill passed by 313 votes to 312, another vote had been tied – the first time such a situation had occurred in the Commons since 1993.
The tie, about whether MPs could again seize the Commons’ order paper next week, was broken by Speaker John Bercow who voted against the proposal in line with precedent that the Speaker should not endorse a policy change which does not have a majority in the Commons.
The DUP’s 10 MPs voted against the bill, along with 290 Conservative MPs, nine Labour MPs and three independents.
Lady Hermon voted with The Independent Group, the SNP, the Liberal Democrats, Plaid Cymru, the Green Party, 14 Tory MPs and 229 Labour MPs.
Lady Hermon, the only pro-Remain Northern Ireland MP to take their seat in the Commons, has consistently spoken in favour of the backstop, arguing that it is necessary to protect the Belfast Agreement.
Speaking during the debate, Lady Hermon said that “it would be an unmitigated disaster for Northern Ireland if this country were to leave without a deal”.
Addressing her comments to former Labour direct rule minister David Hanson, the former Ulster Unionist went on: “It would be an unmitigated disaster in terms of security — he will know all about the threat from dissident republicans, and he will also know that Sinn Féin would use a no-deal Brexit to campaign for a border poll to take Northern Ireland out of the United Kingdom and into a united Ireland.”
The Cooper-Lewtin bill will now be fast-tracked through the House of Lords.
If the legislation passes on to the statute book, it would require the prime minister to table a motion seeking MPs’ approval for an extension to the Article 50 process beyond April 12 to a date of her choosing.