The procedure was last invoked in 1990 when Sir Paul Dean, the then Deputy Speaker, used his casting vote to prevent a last-minute concession during the Human Fertilisation and Embryology Bill, which would have made it more difficult to get late abortions.
Miss Boothroyd, Speaker of the House of Commons, and a former Labour MP, who used her casting vote to support the Government in defeating the Social Chapter amendment, is no stranger to being in the parliamentary limelight.
Senior parliamentarians have been increasingly anxious that there was a ministerial whispering campaign in the wake of the way she handled the resignation of Michael Mates as Minister of State for Northern Ireland. Her attempts first to silence him and then to back down and let him continue generated some of the worst publicity a modern Speaker has been exposed to.
Some MPs have also expressed concern at the unsubtle tactics she has used to curb long-winded ministerial replies in the chamber. She has taken to placing a hand over her mouth and miming a yawn.
Miss Boothroyd, a former Tiller girl, was elected Speaker last year in only the sixth contested election since 1800, largely because of a Conservative split. Seventeen members of the current Cabinet voted against her.
Last night she followed parliamentary procedure to the letter. If both sides record the same number of votes and further discussion is impossible or not appropriate, the Speaker's casting vote on an amendment to a Bill should leave the Bill in its existing form.