A legal bid to force the UK to hold a referendum on the EU reform treaty has been launched by spread-betting millionaire Stuart Wheeler.
The Conservatives want a referendum on the treaty
Mr Wheeler, a prominent Conservative Party donor, told the BBC he had issued a "letter before the claim" to Prime Minister Gordon Brown.
Mr Brown has rejected a public vote on the treaty as he says it does not change the UK constitution.
But Mr Wheeler says he wants a judicial review of the PM's decision.
He told BBC Radio 4's Today programme he supported the campaign for a referendum but said he felt a legal challenge might be the best way of forcing one.
Mr Wheeler has sent the letter to Mr Brown giving notice of the claim and expects to have a court hearing to decide on his request for a judicial review.
He said he expected to get permission for the review and had been told that the EU treaty - now known as the Lisbon Treaty - could not be ratified while a review was pending.
Mr Wheeler described his legal challenge as "a very serious attempt to get a referendum".
If he is granted permission to apply for a judicial review, Mr Wheeler will claim voters had a "legitimate expectation" that a referendum would be held after one was promised on the EU Constitution.
The government will have 21 days to respond to this before the case comes before a judge.
Breach of contract?
Judicial reviews can last for more than a year, although Mr Wheeler said he expected the government to respond quickly in order to "expedite" the process.
There has never been a case of anyone successfully challenging a government's manifesto pledge in court.
Mr Wheeler said he had originally intended to claim the government was illegally handing powers to Brussels - but had been advised he stood a greater chance of success if he challenged the government over allegedly breaking its manifesto commitment.
European Council president, who will serve for two-and-a-half years rather than countries taking six month turns
New post combining the jobs of the existing foreign affairs supremo and the external affairs commissioner
Smaller European Commission, with fewer commissioners than there are member states, from 2014
Redistribution of voting weights between member states
New powers for European Commission, European Parliament and European Court of Justice
Removal of national vetoes in a number of areas
He said believed he had an "excellent" chance of gaining a referendum but he would also be happy for his legal bid to delay ratification.
In a separate case, to be heard at Brighton County Court on 7 February, former Labour activist Stuart Bower, now a member of the UK Independence Party, is claiming the government broke its promise to hold a referendum on the European Constitution.
The court will have to decide whether the government's refusal to hold one is a breach of contract with Labour voters at the 2005 general election.
The legal bid comes as MPs begin a 12-day debate on whether to ratify the Lisbon Treaty, signed last month by EU leaders.
The government is promising line-by-line scrutiny of the document, but the Tories and Lib Dems say more Commons time needs to be set aside.
Home Secretary Jacqui Smith, opening debate on the justice, migration and home affairs aspects of the treaty, said it would "speed up extradition, strengthen our borders and improve asylum negotiations".
It would also give more protection for children and new ways of sharing information "vital to our efforts to tackle terrorism and serious crime", she said.
"Our citizens are safer and our country more secure from our active involvement in the EU," she added.
But Conservative shadow attorney general Dominic Grieve said the treaty had the potential to "undermine the UK's criminal justice system".
He asked why if the government thought the Lisbon treaty reforms were "such a positive move" it had to negotiate "so many opt-outs and opt-ins?"
As debate continued, Gordon Brown met Nicolas Sarkozy of France, Angela Merkel of Germany, Romano Prodi of Italy and European Commission president Jose Manual Barroso to discuss the global economy.
The Lisbon Treaty replaces the European Constitution, which was rejected by voters in France and the Netherlands in 2005.
All 27 EU countries will have to ratify the treaty before it can come into force.
The treaty contains many of the reforms outlined in the constitution - including changes to voting rights and the creation of a European Commission president - but drops the name "constitution", a reference to EU symbols and an article on the primacy of EU law.
On Monday, the government won a Commons vote for a 12-day debate on ratification by a majority of 56. The opposition had wanted 18 days.