Plans for a supreme court and a limit to asylum seekers' legal rights face a rocky ride after being savaged by the most senior judge in England and Wales.
Lord Woolf has attacked plans for constitutional change
Lord Woolf says plans for a supreme court to replace the House of Lords would be a "second class" institution.
He criticised plans to limit asylum seekers' rights of appeal as "in conflict with the rule of law".
Ministers are pressing ahead with the changes but an opposition peer says it could force a constitutional crisis.
The government insists it is responding to voters' concerns by shaking up the judiciary and by speeding up the asylum process, and says it will not be cowed.
Liberal Democrat peer and QC Lord Goodhart said some judges had been talking about a "nuclear option" on the asylum plan where they would hear appeals despite being legally barred from doing so.
"Then he would be into a really major constitutional crisis and I certainly hope we will not see the need for that," he told BBC Radio 4's World At One.
Lord Woolf on Wednesday evening said plans to prevent access to the courts over asylum decisions could fuel a campaign for a written constitution.
He told Cambridge University's law faculty the asylum proposals were a "blot on the reputation of ministers" and derided Lord Chancellor Lord Falconer as a "cheerful chappie".
He argued: "Immigration and asylum involve basic human rights.
"What areas of government decision-making would be next to be removed from the scrutiny of the courts?
"What is the use of courts if you cannot access them?"
Separation of the powers
His speech also tackled the government's broader constitutional changes.
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.
Lord Falconer was described as a "cheerful chappie"
Lord Woolf said the reforms could lead to too much power residing with the Home Office.
A spokesman for Lord Falconer's department defended the government's plans for constitutional reform.
"These proposals were only brought to Parliament after extensive consultation and agreement with senior members of the judiciary, including the lord chief
justice, the legal professions and the public," he said.
"Central to the bill is the principle that politicians have no place in the courtroom and judges have no place in Parliament.
"This separation of powers is
vital to maintain public confidence in political institutions."
A Home Office spokesman said that if Lord Woolf had constructive suggestions to make they would be happy to hear them.
"If we had been cowed by previous criticisms we would never have halved asylum claims or got life meaning life for murderers."
The reforms are set to face stiff opposition in the House of Lords.
Conservative Lords Leader Lord Strathclyde said his party would back a motion on Monday to put the reforms on hold until they were examined by a special select committee.
Former Master of the Rolls Lord Donaldson on Thursday backed Lord Woolf's criticisms, saying the new supreme court would not be like those seen in other countries as it could not strike down laws.
He told BBC Radio 4's Today programme the asylum change was unacceptable.
"The government is saying: 'Well look, in this area where there is a good deal
of popular pressure we will not allow you to do your ordinary job of
safeguarding the rights of the citizen at all, you must take gardening leave'," he added.