The Tories have put forward amendments to government legislation calling for a referendum on any future treaties transferring powers from the UK Parliament to the EU.
As MPs continued scrutiny of the Constitutional Reform and Governance Bill at committee stage on 19 January 2010, shadow Europe minister Mark Francois said the aim was to "give the British people the final say on whether they wish this Parliament to hand over further areas of power to the EU".
Mr Francois pledged that if they failed tonight, but won the coming General Election, the Conservatives would seek to amend legislation then to achieve the "referendum lock".
But Labour backbencher and former Europe minister Keith Vaz warned that such proposals would result in a "constant state of referendum".
If the court system became involved in determining whether a treaty had involved a transfer of power from the UK to the EU, the courts would be "completely log-jammed" - and then there was "no point in having a Parliament", he declared.
For the Liberal Democrats, Ed Davey said he would not support the Tory amendments as they were "fatally flawed". Referendums would constantly be held on "minor issues of policy", he warned.
Europe Minister Chris Bryant dismissed the Tory amendments as "impracticable, wrong and unnecessary".
He accused Tory leader David Cameron of tabling the proposals simply to appease the Eurosceptic wing of his party.
When MPs divided, the move was defeated by 303 votes to 183, a government majority 120.