KIDNAPPERS holding Briton Kenneth Bigley last night threatened to execute him "if the British Government doesn't do what it has to".

The threat came in a statement posted on an Internet website less than an hour after the captors announced they had executed the remaining American hostage, following the murder of Eugene Armstrong on Monday.

Construction contractor Mr Bigley, 62, is the only captive remaining in the hands of the Tawhid and Jihad militant group following the apparent beheading last night of colleague Jack Hensley.

The three men were seized by the group, led by the notorious Abu Musab al-Zarqawi, from their home in the wealthy al-Mansour district of Baghdad last Thursday.

The hostage takers are demanding the release of Iraqi women from specified jails.

On Monday, horrific video footage showing the beheading of Mr Armstrong was broadcast on a website, and his body was later recovered.

On a website last night, an apparent message from the kidnappers stated: "The nation's zealous children slaughtered the second American hostage after the end of the deadline. We will provide you with film of the slaughter soon, God willing."

The statement was signed in the pseudonym Abu Maysara al-Iraqi, who has posted statements in the group's name in the past.

Later, a statement was posted on another website, which said: ''The British prisoner will get the same fate if the British Government doesn't do what it has to. The blood of Muslims is not water! The honour of Muslim women will not go to waste.''

In London, the Foreign Office said it could not confirm reports of the second execution.

But to Mr Bigley's family, news of Mr Hensley's apparent murder was further agonising proof of the ruthlessness of the kidnappers.

Yesterday, they had appealed to the Prime Minister to intervene and save the engineer's life.

The Government made a fresh appeal on Arab television for information on the captives and to stress it was impossible to meet the kidnappers' demands.

Afterwards, a Foreign Office spokesman said: "We just cannot. It would be open season for terrorists."

The US said it was holding two females, but not at either specified prison. They are believed to be Dr Rihab Taha, head of the Iraqi biological weapons programme and known as Dr Germ, and biologist Dr Huda Salih Mahdi Ammash, nicknamed Chemical Sally or Mrs Anthrax.

Mr Bigley's brother, Philip, begged the premier to help, adding: ''The death of the American hostage has proved to us that if nothing is done, then the two remaining captives will die by the most horrific means.''

The three captured men were working for Middle East-based Gulf Supplies and Commercial Services, where colleagues yesterday revealed Mr Bigley was days from retirement.

Last night, his elder brother, Stan, 67, had travelled from his home in Wigan, Lancashire, to wait for news at their mother's home in Walton, Liverpool.

A female family member said at the house in Wigan: "All the brothers are together, waiting for the worst."