Vince Cable was prevented from asking a Commons question
Lib Dem deputy leader Vince Cable has attacked "ridiculous" Commons procedures after he was banned from asking a question about the Queen.
Mr Cable mentioned the Monarch in a question on the economy to Harriet Harman, who was standing in for Gordon Brown at prime ministers questions.
But he was prevented from completing it by Speaker Michael Martin.
Mr Cable said it was "ridiculous" MPs could not mention the Queen in passing "without prior permission".
In the Commons, Mr Cable was cheered by MPs when he rose to ask the first of his permitted questions to Ms Harman.
He said: "It was reported this week that Her Majesty the Queen had cancelled her diamond wedding celebrations because it was judged to be inappropriate to engage in extravagance at a time of economic gloom and recession.
"Do you share my view that this demonstrates Her Majesty's unerring instincts for the public mood, or does the Government think she was overreacting?"
But Speaker Martin would not let the question be answered.
He ruled: "Order! You shouldn't discuss Her Majesty the Queen. Perhaps you can try another question. He has used one up. He can try another one."
Mr Cable said he was very happy for the Labour deputy leader to "return to the issue of economic gloom and recession and whether you share that assessment."
Outside the Commons he said: "It is absolutely ridiculous that in a supposedly modern democracy Members of Parliament cannot even mention the Head of State in passing without prior permission.
"This is yet more proof of the major constitutional reform needed to drag Westminster into the 21st century."
Commenting on Mr Cable's decision to mention the Queen, former Lib Dem leader Sir Menzies Campbell said: "I think he made a mistake, but people make mistakes at prime minister's questions."
He added: "I'm sure he will be back at his pungent best."
It is a Commons convention that MPs do not discuss the Queen.
'Sent to the Tower'
According to Erskine May, the guide to parliamentary practice, "the irregular use of the Queen's name to influence a decision of the House is unconstitutional in principle and inconsistent with the independence of Parliament".
It adds: "Any attempt to use her name in debate to influence the judgement of Parliament is immediately checked and censured."
It says MPs have been reprimanded "or even sent to the Tower" for treasonable or seditious language "or disrespectful use of Her Majesty's name".
Lib Dem leader Nick Clegg has vowed to highlight what he sees as antiquated Parliamentary procedure as part of his campaign for democratic reform.
He has said Parliament "is fast becoming a museum piece - a 19th-century home for our 21st-century political elite".
Last month Lib Dem frontbencher Ed Davey was thrown out of the Commons for protesting against the Speaker's decision not to allow a vote on a referendum Britain's continued membership of the EU.
Fellow Lib Dem MPs walked out of the chamber in solidarity during debate on the Lisbon Treaty.
Mr Clegg told delegates at the party's spring conference in Liverpool they could expect to see further such protests.