The Public Health Bill has been defeated in the Senedd after the final vote on the proposed law was tied with 26 votes for and 26 against.
The Presiding Officer was then required to use her casting vote to halt the legislation.
The Welsh Government, which has no majority in the Assembly, had been relying on the support of Plaid Cymru AMs to pass its proposals, which included restrictions on the smoking of e-cigarettes.
Relations between the two parties broke down this afternoon following a row over earlier legislation.
The defeat was the final act before the Presiding Officer, Dame Rosemary Butler, announced the end of the 2011-2016 Assembly.
It won't meet again before it's dissolved next month, prior to the election in May.
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