A fourth suspected terrorist from Crawley has been arrested as police investigate an alleged bomb plot.

The 27-year-old man was taken into custody last night the ninth British citizen to be held since the high-profile operation began on Tuesday.

He was taken to Paddington Green police station in central London where officers were yesterday granted another 72 hours to question the other arrested suspects, all Muslims aged from 17 to 32.

Seven of the nine are believed to have links to Crawley.

Relatives of Ahmad Kan, 18, Omar Khyam, 22, and Shujah Khyam, who all live in the Langley Green area, have protested their innocence.

In the latest case, the man was arrested on suspicion of being concerned with the commission, preparation or instigation of acts of terrorism.

His arrest followed another day of intense police activity in the town.

A number of houses remained under police guard while Home Office scientists in protective suits continued their searches.

The operation has moved across the Atlantic with the arrest of Mohammad Momin Khawaja, 24, from Ottawa, Canada, by Canadian police.

His arrest led to fears that the tentacles of the alleged plot in Britain could have stretched across the globe.

At the time of his arrest, Khawaja was working for the Canadian government's department of foreign affairs.

His family said Khawaja's father, Mahboob Khawaja, 62, an international affairs expert, had also been arrested and was being quizzed by authorities in Saudi Arabia where he was teaching at a college.

The elder Khawaja has published a number of books and papers critical of western influence on Middle East politics since going to Canada more than 30 years ago.

Police in Sussex are examining computers, including some seized from an internet cafe in the Langley parade shopping precinct, to see if the British suspects were communicating with al-Qaida operatives abroad.

Like forces in other countries, British police are also in contact with Spanish police who have named six suspects for the Madrid railway bombings last month.

The British operation follows the discovery of half a tonne of ammonium nitrate fertiliser, a key ingredient in the manufacture of explosives, at a west London warehouse. MI5 and police moved in after months of covert surveillance.

Members of the Crawley mosque, where the three used to regularly visit before their arrest, called off a Press conference at the last minute yesterday.

Reporters invited to the mosque's car park were asked to leave while those inside had talks with police.