Eurosceptic MPs will renew calls for a referendum on the Lisbon Treaty when it comes up for debate again in parliament in the coming weeks.
A new amendment to the treaty – expected to be rubberstamped by EU leaders in June – will require ratification at Westminster.
The Foreign Office says it is only a technical adjustment.
But some Tories are set to use the process to call for a vote, despite David Cameron already ruling one out.
National sovereignty
The Lisbon Treaty, a substitute for the rejected European constitution, is often described as an attempt to streamline EU institutions to make the bloc of 27 states function better.
But its opponents see it as part of a federalist agenda which threatens national sovereignty.
Many people had thought the treaty was all wrapped up when the Czech president signed it last November.
But MEPs have agreed an amendment adjusting the composition of the European Parliament, which held elections last year before the treaty came into force.
The amendment allocates 18 extra MEPs to 12 countries, including one extra MEP for the UK . It will require approval throughout the EU.
According to the Foreign Office, the adjustment would not transfer any new powers to Brussels.
Last November, the then-shadow foreign secretary William Hague said the Conservatives would not hold a referendum on the treaty.
The announcement prompted accusations from Eurosceptics in his party that he had reneged on a "cast iron" guarantee made in 2007 to hold a referendum on any treaty that emerged from EU talks.
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