France's decision to reject the EU constitution has thrown the UK's plans to go to the polls in the next 18 months into uncertainty. Here is a guide to the debate and where the key players stand.
What is the European Union constitution?
It is essentially a rule book setting out what the EU can and cannot do. It sets down in writing the EU's values and political objectives.
What is it for?
The constitution is designed to streamline decision-making in the enlarged EU of 25 states. For example, it lowers the size of the majority needed for most decisions in the European Council, and cuts the number of areas where a unanimous vote is required.
But it also opens the way to deeper EU integration and greater centralisation of decision-making.
Opponents argue the constitution will hand control of important issues such as foreign affairs, public services and the economy to politicians not elected by British voters.
Who supports it?
Prime Minister Tony Blair and Labour's front bench, the Liberal Democrats, Plaid Cymru and a small group of Conservative MPs including leadership contender and former Chancellor Ken Clarke.
Mr Blair is a relatively recent convert to the idea of an EU constitution, previously preferring to talk about a "charter of competencies".
He has argued the constitution is in Britain's interest and claims to have secured a good deal in treaty negotiations, protecting the UK's vetoes on economic policy, defence and foreign affairs.
The Scottish National Party (SNP) is in favour of closer European integration, but says it can not back a constitution which "claims fishing policy as an exclusive competency of the European Union".
The Scottish Greens support the idea of a constitution, but "have grave concerns over the current treaty".
Britain in Europe, a cross party pro-EU lobby group, backed by some of Britain's biggest companies, is likely to spearhead the yes campaign, when a referendum is called.
Who is against it?
Conservative leader Michael Howard and most Tory MPs, The UK Independence Party, the Green Party, Respect and about 70 backbench Labour MPs.
The Conservatives want a new changes that would shift some powers back to nation states.
UKIP believes rejecting the EU constitution will move Britain a step closer to its aim of withdrawal from the EU.
The No Campaign, which was officially launched in May 2005, claims cross party support at Westminster, including Labour former minister Kate Hoey, and the backing of more than 550 business leaders, trade unionists, economists and activists, including former UN ambassador Sir John Weston, Green MEP Caroline Lucas and Next chief executive Simon Wolfson.
Who else is having a referendum?
Ten other EU countries have so far said they will hold referendums.
Spain became the first country to approve the constitution by referendum on 20 February. The upper house of parliament has yet to complete ratification.
The constitution was backed by 77% of voters, with 17% against in the referendum, which was not legally binding. With all of Spain's main political parties in favour, ratification is all but guaranteed.
A further nine other countries have ratified the treaty, they are: Austria, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Slovakia and Slovenia.
France voted "No" in its referendum on 29 May. The Netherlands did likewise, with 61.6% of voters saying "No" on June 1.
What does this mean for the UK vote?
Originally, Tony Blair said he would hold a British vote on the treaty, expected during 2006. However, when France rejected the treaty on 29 May, he said it was too early to say whether Britain would hold its referendum.
Instead, Mr Blair called for "time for reflection".
After the Netherlands rejection on 1 June, Foreign Secretary Jack Straw said there were "profound questions ... about the future direction of Europe".
Mr Straw will make full statement to the House of Commons on Monday 6 June on the impact of the referendums. He is expected to announce that the Bill to allow a referendum in the UK will be shelved.
The government is not saying publicly that the latest two "No" votes kill the constitution - it says no one country can declare the treaty dead.
But ministers say the votes leave the constitution in "serious difficulty" and Mr Blair will discuss their impact with fellow European leaders at a summit in Brussels on 16 June.
Does anyone still want a referendum?
The Conservatives say the treaty dead, but they still want a UK referendum if any of the changes proposed in the treaty are introduced in some other way. This view is shared by the No Campaign group.
The Liberal Democrats argue there is now no prospect of a referendum, a feeling shared by the cross-party pro-Europe group Britain in Europe.
Scottish National leader Alex Salmond said it would be a sign of "arrogance" if Britain pressed ahead with a referendum.
The UK Independence Party believes the UK should now hold a vote on whether or not to remain a member of the European Union.
This opinion is echoed by the Eurosceptic newspapers.
The Daily Mail argues it is in the public's interest for the prime minister to "honour his pledges and allow the British people their first referendum on matters European for 30 years".
The Sun adds that shelving the referendum would mean British people "will be denied the right to decide their own futures".
Unless the UK gets a chance to "vote no", the paper warns, "there will always be scope for EU power brokers to strike deals and saddle us with a watered down treaty".
But other newspapers argue a referendum is pointless.