By Andrew Osborn

LONDON (Reuters) - Muslim women should be banned from wearing a veil when giving evidence in British courts, a cabinet minister in Prime Minister David Cameron's government said on Sunday, arguing it was hard to judge someone's testimony otherwise.

In comments likely to stir up an already emotive debate, Ken Clarke, a minister without portfolio who used to work as a criminal barrister, likened traditional female Islamic dress to being "in a kind of bag", and said he found it "a most peculiar costume for people to adopt in the 21st century."

"I think we do need a clear rule. I don't think a witness should be allowed to give evidence from behind a veil," Clarke, a former Home Secretary, told BBC radio.

"I can't see how on earth a judge and a jury can really appraise evidence when you're facing someone who is cloaked and is completely invisible to you. It's almost impossible to have a proper trial if one of the persons is in a kind of bag."

A judge's ruling in September that a Muslim woman could not give evidence at her trial wearing a full-face veil sparked debate about whether Britain should follow other European countries and ban veils in schools and public places.

Judge Peter Murphy said at the time he hoped parliament or a higher court would provide a definitive verdict "sooner rather than later".

Britain has so far steered clear of following the examples of France and Belgium, where it is illegal for women to wear full-face veils in public.

Clarke, who said he had no objection to anyone wearing what they liked outside the courtroom provided it was "decent", said it was vital for jurors to be able to observe a person's body language and facial expression to make a decision on whether they were telling the truth.

Face-coverings were therefore an obstacle to justice, he said.

"I actually think it undermines a trial and that's not based on any trace of islamophobia."

Cameron's government is considering how to better integrate Britain's 2.7 million Muslims without restricting their right to freedom of religious expression.

(Editing by Sonya Hepinstall)