Kate McCann has refused to take a lie detector test about her daughter Madeleine's disappearance, it was revealed yesterday.

She and husband Gerry had offered to undergo a polygraph examination in September, after they were made official suspects in the investigation.

But it has now emerged that they have refused an expert's offer to carry it out, because the results would not be admissible as evidence to a Portuguese court.

Don Cargill, chairman of the British and European Polygraph Association, said the McCanns told him they would only take the test if it was 100 per cent accurate and admissible in a Portuguese court.

He told the Sunday Express: "Kate said she'd take it to prove her innocence but in reality, she wasn't willing.

"I was dumbfounded, to tell the truth.

"I don't think it was the McCanns' fault. I was left with the impression the whole thing was a PR exercise to get sympathy at a time when Kate was under increasing scrutiny."

Lie detectors work by measuring physiological responses such as blood pressure levels, pulse rate, breathing and sweat gland activity in the skin during questioning.

Any significant difference in these rates may indicate the subject is lying.

The process has been criticised but the American Polygraph Association claims the current computerised technology is 98 per cent accurate.

They are not admissible in British or Portuguese courts.

McCann spokesman Clarence Mitchell said: "Of course they are not going to take the test. It's inadmissible in Portugal and there are doubts about the accuracy.

"Gerry and Kate don't need to do one as they are telling the truth."