They are seen as a mark of poor communication, and can be. . . um. . . simply maddening for anyone waiting for a punchline.

But the utterances that slow down our sentences actually make us better understood, according to scientists.

They found that "ums", "ers" and "ahs" - known as "disfluencies" - force a listener to pay attention.

The experts at Stirling and Edinburgh universities asked volunteers to listen to a series of sentences, including a number which had disfluencies.

They then carried out a series of tests to measure how much the listeners could remember.

Inserting the "ers" apparently had a significant effect on how well the subjects could recall what was said.

Up to an hour after hearing typical sentences, volunteers got 62 per cent of words correct where there had been a disfluency in the sentence.

That compared to 55 per cent for sentences where there had not been any stumbles, the team said in their report, It's The Way That You, er, Say It.

Dr Martin Corley, of Edinburgh University's School of Philosophy, Psychology and Language Sciences, said: "A disfluency is an interruption to the predicted ritual of things.

"It's like we are saying to ourselves 'I'd better pay attention now, because what I thought was going to happen isn't going to happen'."

The results fly in the face of current thought on um-ing and ah-ing.

Broadcasters, politicians, lawyers and the clergy, for example, are advised to iron out disfluencies from their speech.

Neil Stevenson, deputy director of education and training at the Law Society of Scotland, said: "We're sure some of our members will be delighted that their ums are now seen as a sign of clear communication."

The researchers are now studying whether words such as "like" operate in a similar way to disfluencies.