THE family of British hostage Kenneth Bigley continued their wait last night as Iraqi authorities backtracked on an apparent concession to his kidnappers' demands.

It had been thought that one of the two high-profile female prisoners in US custody would be released shortly, giving a glimmer of hope to the family.

But Qassim Dawoud, a security advisor to interim Iraqi Prime Minister Ayad Allawii, said weapons scientist Dr Rihab Taha, nicknamed Dr Germ, would not be freed immediately.

Militants holding Mr Bigley - who are now believed to have beheaded both of his American colleagues - threatened to kill the 62-year-old British engineer unless Iraqi female prisoners held by the Americans were released.

Mr Dawoud said of Dr Taha and two other prisoners who are on the US's most-wanted list: "Until now the security measures are still going on. I say they will not be released today, tomorrow or after tomorrow - but after they undergo a medical check-up and security measures."

The US Embassy in Baghdad said Dr Taha and the second woman being held - the microbiologist Dr Huda Salih Mahdi Ammash, known as Mrs Anthrax - would not be released immediately.

Iraq justice minister Malik Duhan al-Hasan had previously announced plans to free Dr Taha on bail and said this was not linked to the hostage-taking.

The Tawhid and Jihad group boasted on Tuesday night of killing a second American, Jack Hensley, after executing his fellow hostage, Eugene Armstrong.

The Bigley family has endured an agonising wait while the militants beheaded Mr Armstrong on Monday, then reportedly killed Mr Hensley on Tuesday, leaving only the Briton remaining.

Video footage showing the death of Mr Armstrong was broadcast on a website and a body believed to be his was recovered.

A second body and severed head, feared to be Mr Hensley, were found in a black plastic bag in Baghdad's Amiriya neighbourhood earlier yesterday and handed to US forces.

The extremists are said to have restated their intention to kill Mr Bigley, but did not give any deadline.

His brother, Paul, recorded another message to the kidnappers, to be broadcast on Arabic language TV station al-Jazeera, urging them to release his brother in response to the planned release of Dr Taha.

He said the Iraqi government's announcement that the woman would be freed was "a point in the right direction", and urged the Baghdad authorities to release the second woman.

Speaking from Amsterdam, Mr Bigley told BBC Radio 4's Today programme: "They need to see it on television, they need to see females walking free.

"Hopefully, they will pick this up on the media and show that they have a gram of decency in them by releasing Ken.

"I hope that my last message to them, telling them about these two ladies, recorded half an hour ago, will get through."

He said governments should be ready to negotiate with the hostage-takers.

"If you have a conflict, you have to sit down and some people have to give, some have to take - you have a compromise," he said.

Speaking of Mr Hensley's apparent murder, he said: "It is absolutely horrendous for members of the family and for all concerned."

Mr Bigley said he had wanted to call for the release of the female prisoners from the moment his brother was kidnapped, but had been suppressed from speaking out.

Asked if he had been given any information by the Government about measures to secure his brother's release, he said: "The Government aren't saying anything. They are keeping tight-lipped about the whole thing."

Mr Bigley and the two Americans were seized from the garden of their home in the al-Mansour district of Baghdad without a struggle last Thursday