By Joshua Rozenberg
BBC Radio 4 presenter
Lord Neuberger believes the change will have unintended consequences
Britain's Supreme Court could become more powerful than the House of Lords committee it replaces next month, a leading judge has told the BBC.
Lord Neuberger fears the new final court of appeal could assert itself in opposition to the government.
He says there is a real risk of "judges arrogating to themselves greater power than they have at the moment".
The new court president's Lord Phillips said such an outcome was "a possibility", but was "unlikely".
Lord Neuberger has stepped down from the Law Lords and returns to the Court of Appeal next month as head of its civil division with the title of Master of the Rolls.
In an interview for Radio 4's Top Dogs: Britain's New Supreme Court, he suggested that the Supreme Court had been created "as a result of what appears to have been a last-minute decision over a glass of whisky".
"The danger is that you muck around with a constitution like the British Constitution at your peril because you do not know what the consequences of any change will be," he said.
The government announced in 2003 that it would end the constitutional anomaly under which a House of Lords Committee served as Britain's final court of appeal, and replace it with a 12-member Supreme Court located in its own building.
Lord Neuberger's fears were dismissed by Lord Turnbull, who was one of Tony Blair's most senior advisers when the then prime minister announced wide-ranging constitutional reforms.
"It has been alleged that this was all thought up on the back of a fag packet," the former Cabinet Secretary told the programme.
"This is entirely wrong."
Even so, Lord Turnbull accepted that the Supreme Court justices might become more assertive and more difficult for a future government in their new home across Parliament Square.
Lord Falconer, the Lord Chancellor, who saw the reforms through Parliament, agreed that the new court would strengthen the judiciary.
"The Supreme Court will be bolder in vindicating both the freedoms of individuals and, coupled with that, being willing to take on the executive," he said.
But Lord Bingham, a former senior law lord, said there was no question of the Supreme Court justices having a "rush of blood to the head" and "throwing their weight around".
He was supported by the new court's president Lord Phillips.
"I can't predict quite how we are going to function in the new world," he told the programme.
"But I don't myself see that it's likely that we're going to see a change of that nature."
Top Dogs: Britain's New Supreme Court will be broadcast on BBC Radio 4 on Tuesday 8 September at 2000 BST.
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