It could be illegal to allow European institutions to enforce stricter budget rules on member states, a Conservative MP has claimed.
Veteran eurosceptic Bill Cash was speaking during an emergency debate in the Commons on the "legality" of the decision of most EU countries to agree to a new treaty on greater fiscal unity.
The pact - which is not EU-wide - was agreed by 25 member states in January, in a bid to prevent further crises in the economy; the UK and Czech Republic have declined to sign up to it.
Addressing MPs on 29 February 2012, Mr Cash, who is chair of the Commons EU Committee, argued that giving the European Commission and European Court of Justice power over a treaty to bring financial stability to the eurozone would undermine the rule of law.
He said: "The treaty is based on the dangerous assumption that the end justifies the means and that even if it is unlawful, the requirement to bring in the treaty for political reasons overrides the law.
"The question is whether it is lawful for the European Union's institutions such as the Commission and the European Court to be involved in such an agreement.
"The new treaty is the triumph of expediency over the law."
However, the Liberal Democrat's Martin Horwood said he believed Mr Cash had "missed the point" on a "colossal" scale.
He told MPs questions about the legalities of the treaty were "dwarfed" by the "severe" problems facing the European economy, adding that the treaty was a "since and concerted" effort to make the area "stronger" and "more prosperous".
He suggested Mr Cash was putting his own ideology ahead of the country's national interests, and that his obsession with the legal minutiae amounted to "almost the political equivalent of anti-social behaviour".
Labour MP Chris Bryant said the UK faced two choices: make the euro work or dismantle it. He supported the former, arguing that the collapse of the euro was bad for the UK's national interests.
Shadow Europe minister Emma Reynolds attacked the government's "manifestly unclear" position on the subject.
She said David Cameron had vetoed the treaty in December over concerns about the use of European institutions, but the coalition was now poised to allow the 25 nations who signed the treaty to use the EU institutions.
The debate, which had been granted by Commons Speaker John Bercow, came ahead of a European Council meeting in Brussels.
Europe Minister David Lidington, replying on behalf of the government, said David Cameron's decision to wield the veto "ensured that what others decide to do is not binding on the UK through EU law".
He told the Commons that the primacy of EU law could not be affected by inter-governmental agreements.
He added that the UK reserves the right to take legal action if EU institutions are used in a way that was contrary to EU treaties, which was the "best way" of protecting the country's interests.