Downing Street has said it will not be "held hostage" by peers over its plans to abolish the historic post of lord chancellor and set up a supreme court.
Lord Falconer's historic post would be abolished under the changes
Debate on the Constitutional Reform Bill is under way, with Conservatives and some ex-law lords ready to vote to send it to a special committee.
Tony Blair's official spokesman said the move was a delaying tactic to "kick the ball into the long grass".
Critics say the plans are pointless and threaten judicial independence.
The Bill also seeks to eject the law lords from the upper House as well as establish a new independent commission to appoint judges.
The committee of law lords in the House of Lords currently acts as the court for final appeals in the UK.
The proposed reforms would leave that job to a new supreme court working completely separately from Parliament.
The government says that change will protect the independence of the judiciary and set clear boundaries between politicians and judges.
There is a convention that law lords do not vote on controversial issues.
But former law lord Lord Lloyd, a crossbench peer, is pressing for the Bill to be given further scrutiny by a Lords select committee.
He said a closer look was needed at the costs of a supreme court, estimated at between £6m and £32m to set up, and £8.7m a year to run, compared with the £623,000 annual cost of the House of Lords court.
"It's not too late to save the office of Lord Chancellor - that's what I hope we shall
do," he said.
However, ministers argued that the plan would effectively kill the Bill.
If that happens they could withdraw the Bill and use their powers to force the measure onto the statute book next year, they said.
Tony Blair's official spokesman said: "We cannot be held hostage to the Lords on major areas of reform."
There had been four months of consultation on the proposals, he said.
With Liberal Democrats set to back the government and many cross-benchers expected to join forces with the Tories, a close vote is predicted.
Opening the second reading debate, Lord Chancellor Lord Falconer said sending the Bill to a special committee could mean MPs never had a say on the issue.
The current system had worked well in recent years but one failure would undermine it, he said.
"We should change when we are strong, we should recognise that we can improve the system," he argued, rejecting any suggestion the new court would be "second rate".
Tory Lords Leader Lord Strathclyde denied he was wrecking the bill.
He told BBC News 24: "The lord chancellor's post has been with us 1,400 years. All we are asking for is another 14 weeks or so to give the plans the scrutiny they deserve."
Crossbencher Viscount Bledisloe said the bill was "thoroughly ill-thought
out", while Labour former Lords Leader Baroness Jay of Paddington described Lord Lloyd's
amendment as "very unhelpful".
Liberal Democrat spokesman Lord Lester QC backed the principles behind the government's plans, but said the new court should first be allocated a home and enough resources.
Independent Labour peer Lord Stoddart of Swindon described the Bill as "a piece
of constitutional vandalism, undermining the judicial balances built up over
centuries, which still work and have the country's confidence."
New appointments' process
Lord Nolan, a law lord, was worried about the appointments process, saying there was a danger that parts of the Bill could leave judges "subservient" to the government.
Last week, Lord Woolf, the most senior judge in England and Wales, said the plans could mean a first class final appeals court was replaced by a "second class" supreme court.
On Monday, the lord chief justice said he was ambivalent about the idea but if it went ahead the judiciary wanted proper accommodation and resources.
He hoped the Bill could go through the normal scrutiny process instead of a committee because he wanted to see the new judicial appointments process in place.