Calls were made for the ratification of the EU Reform Treaty (the Lisbon Treaty) to be put on hold pending a High Court challenge over the Government's refusal to subject it to a referendum.
The calls came immediately after millionaire businessman Stuart Wheeler, a major Conservative Party donor, won permission from a senior judge to seek a judicial review.
He claims that he had "a legitimate expectation" that Gordon Brown would hold a public vote.
The Prime Minister has refused on the grounds that the Lisbon Treaty, introduced after proposals for a EU constitution were voted down by the French and Dutch in 2005, did not alter the UK constitution.
Rabinder Singh QC, appearing for Mr Wheeler, argued the rejected constitution and the proposed Treaty were the same in all but name.
He said the obligation to hold the promised referendum could not be avoided simply by a name change.
Mr Justice Owen, sitting at London's High Court, ruled that Mr Wheeler, 73 - who made more than £30 million from spread-betting firm IG Index - had "an arguable case" that should go to a full hearing on June 9-10.
Later Open Europe - Mr Wheeler is a board member - called on the Government to halt attempts to ratify the Treaty without a referendum.
The group's chairman Lord Leach said: "The Treaty is now in the House of Lords, where it is receiving the thorough scrutiny denied to MPs by the Government's business managers in the Commons. The Government can - and should - now stay its hand pending the outcome of this judicial review."
Responding to the ruling, a Foreign Office spokesman said: "Hardline Eurosceptics brought similar cases in respect of earlier European treaties, so this case is no surprise. Those challenges all failed. We are confident of the strength of our case on this occasion and look forward to putting our arguments before the court in more detail in due course."
