Labour and the Liberal Democrats have been accused of "conspiring" to block a Tory bid to enshrine in law David Cameron's commitment to a referendum on European Union membership.
Whips from the two parties met yesterday ahead of the legislation's first debate in the House of Lords, where it is expected to face a number of obstacles.
The backbench bill, which has enjoyed high-profile support from the Tory leadership, faces a race against time to make it onto the statute book because of the limited opportunities to debate private members' legislation in both houses.
The European Union (Referendum) Bill has already been steered through the Commons by Tory MP James Wharton, who warned against attempts to block the legislation in the upper house, where it will be debated on Friday.
He said: " Having tried and failed to block the Referendum Bill in the Commons, Labour and the Lib Dems are once again conspiring to stop the British people having a say.
"Fundamentally, Labour and the Liberal Democrats don't trust the British people and once again, the Conservatives will be the only party fighting to get this Bill through and give the British public the referendum they deserve.
"Labour and Lib Dem peers must not block this Bill, which has passed through the elected House of Commons and the only purpose of which is to give the British people a vote on our membership of the EU."
The legislation, which will write into law the Prime Minister's pledge to hold an in/out referendum on EU membership by the end of 2017, will be taken through the Lords by Tory peer Lord Dobbs.
In order to complete its passage in this parliamentary session, the Bill will need to have progressed through the Lords and returned to the Commons by the end of February.
A significant number of amendments are expected to be tabled by backbench peers, which could delay the legislation's progress, and any changes which are made could derail it entirely because they would need to be agreed by MPs - taking up valuable time.
At least 74 peers are expected to take part in Friday's debate, including Tory former Cabinet ministers Lord Howe and Lord Lawson and Labour former business secretary Lord Mandelson.
A Labour source in the Lords said: "This is a significant constitutional bill that deserves careful scrutiny, and as such we'll be guided by the recent reports from the Delegated Powers and Constitution committees.
"A private member's bill isn't the right way to go about major constitutional change - that should only be done with a government bill in government time."
A Liberal Democrat spokesman said: "We would not comment on any meetings held between whips. Whips of all parties meet regularly to discuss a range of issues.
"On the European Union (Referendum) Bill, Liberal Democrats have already voted for the legislation this Government brought forward, which for the first time will guarantee a referendum takes place whenever there is a further change in the European treaties which affect the UK. That is what we promised in our manifesto.
"That said, peers will wish to give the private member's bill the appropriate level of scrutiny when it arrives in the Lords on Friday."