A college principal, who may be a danger to others after receiving intensive radiation treatment, is believed to be on the run in Ireland after failing to appear in Britain on child pornography charges.

A court in London was told that Thomas Leopold (42) could harm anyone he comes into contact with because he was given large doses of radiation for a thyroid condition and his levels of radioactivity could be dangerous to others.

The lecturer jumped bail to board an overnight ferry to Ireland under the noses of British police earlier this month.

"This is not a joke," Judge John Price said as he issued an arrest warrant. "Please warn officers that when he is arrested, he might be radioactive."

Mr Leopold is thought to have been given radioactive liquid iodine to treat his ailment. The radiation eventually fades but patients can still pass it on through urine, saliva and sweat.

Mr Leopold, who is head of a tutorial college in Harrow, Middlesex, is accused of downloading indecent images of children in 2006.

His trial had been listed for hearing on February 9 but only four days earlier he was questioned by a Special Branch officer as he attempted to board a ferry in Fishguard in Wales.

"He told the officer his bail condition had been varied, enabling him to make the journey to visit his mother," prosecuting attorney Nick Mather told Southwark Crown Court.

Mr Leopold was then waved on to the ferry bound for Rosslare and has not been seen since. It is understood that when he showed his radiation card, he was permitted to stay in his own car as the ferry travelled across the Irish sea.

"He is in chronic poor health," defending attorney Jeannie Mackie told the court.

"His doctor confirmed that he is dangerous, in terms of radioactivity, for a period of six weeks after treatment and he had treatment on February 3.

Mr Leopold, from Hammersmith in west London, denies five counts of making indecent images of children and one of possessing 87 such images on or before April 2006.

When he failed to turn up at a previous hearing, he told authorities he was in Ireland looking after his sick mother.