THE IRA last night admitted planting the bomb beneath Hammersmith Bridge which failed to detonate properly three days ago.

A statement issued to Ireland's RTE television and radio network said: ``The substantial explosive device, which unfortunately failed to detonate, at Hammersmith Bridge in London, was placed by one of our active service units.''

The claim, accompanied by a recognised code word, was made after anti-terrorist police investigating the IRA's renewed campaign in the capital arrested several people in dawn raids yesterday.

Armed officers swooped on addresses in Fulham, west London, and Essex shortly after day-break.

The arrests were made under the Prevention of Terrorism Act and those detained were taken to a central London police station.

They were being questioned about the three most recent terrorist incidents in the capital - the Hammersmith bomb and the small explosions on April 17 at The Boltons in Earl's Court and on March 9 in Old Brompton Road, Fulham.

Police sources were cautioning against interpreting the operation as a major breakthrough in the hunt for the IRA bombers responsible for the renewed mainland campaign.

Those arrested were not thought to be strong suspects, they stressed.

Scotland Yard did not specify how many people were detained but there are believed to be no more than five.

During the raid, police sealed off Britannia Road and officers moved in just after 6.30am. Forensic experts searched a white terraced house at the Kings Road end of Britannia Road.

In Wednesday's incident at Hammersmith, two small explosions, thought to have been caused when detonators went off, followed an IRA warning call to a news agency.

The two devices failed to explode and there were no injuries or damage.

Scotland Yard later revealed the twin devices, placed at the south side of the bridge, together contained upwards of 30lbs of sophisticated explosives, almost certainly Semtex - probably the biggest high-explosive bomb planted by the IRA on mainland Britain.

Successful detonation would have caused massive destruction on the 80th anniversary of the Easter Rising in Dublin of April 24, 1916 - a key date in the Republican calendar.

On Wednesday last week, a small explosion rocked an empty house at The Boltons moments after a coded message was passed to a news agency. Again there were no injuries.

In the early hours of March 9, a ``small, improvised device'' exploded in Old Brompton Road, Fulham, causing minor damage. The IRA later claimed responsibility for the device.

The Hammersmith incident was the sixth IRA attack in the capital since the terrorists signalled the end of their ceasefire with the massive Docklands bombing on February 9.

Meanwhile, a hoax bomber, who telephoned a series of warnings claiming a theatre was about to be destroyed by a massive IRA device, was jailed yesterday for four years.

Christopher Harrison, 33, pretended he had planted 100lb of Semtex in a bin at the side of the capital's Coliseum - just a day after a terrorist explosion ripped apart a bus in London's Aldwych, killing the bomber.

During his final call, staff kept him talking long enough for him to be traced to a nearby phone box.

Passing sentence at Southwark Crown Court, Judge Gerald Butler QC, who jailed Harrison for 27 months for a similar offence in 1992, said conduct such as his caused ``great inconvenience, anxiety, and distress to the public''.

Mr Andrew Colman, prosecuting, said Harrison's first phoned warning on February 19 claimed a device had been left in a black bag next to the building in St Martin's Lane.

The barrister said Harrison, of Cheltenham Place, Acton, west London, phoned four times, insisting a bomb would explode.

However, in his final call, he ``rambled'' on for 15 minutes, while staff and police searched the area.

Interviewed by police, he claimed he was schizophrenic, and that ``voices'' told him to make the calls after he stopped taking his medication.

The court was told a psychiatrist who examined Harrison could find no trace of mental illness.