An elderly couple have been arrested after a package claiming to contain deadly anthrax was sent to Tony and Cherie Blair.

Officers from Scotland Yard's anti-terrorist squad swooped in Littlehampton and arrested them after a surveillance operation spanning several months.

The 63-year-old man and 71-year-old woman have been bailed after their arrest last week pending a full investigation. The package was intercepted before it reached 10 Downing Street and was found to contain powder which later turned out to be harmless.

A Government insider said: "Unfortunately, we get a fair number of hoax packages and letters like this."

The package is understood to have been sent last month and detectives eventually traced the source to Sussex.

The Metropolitan Police had been monitoring correspondence from the Littlehampton area to Downing Street for several months. The couple are due to return to an undisclosed police station in West Sussex next month pending further inquiries.

They face prison terms of up to seven years if charged and then convicted under the Anti Terrorist and Security Act 2001, passed soon after the September 11 attacks on the US.

The insider told The Argus: "They clearly have it in for the Blairs but the couple, I understand, have mental health issues."

The merest hint of a possible terror-related security breach is taken extremely seriously by Downing Street, even if it turns out to be a hoax.

Fathers' rights campaigner Ron Davis, from Findon near Worthing, hit the headlines in 2004 when he and Guy Harrison, from Steyning, threw three condoms filled with purple flour over the Prime Minister in the House of Commons.

The stunt brought Prime Minister's Questions to a halt and sparked fears the House of Commons was under biological attack.

The chamber was evacuated and police, thinking it was an act of terrorism, were sent to the building in protective suits and gas masks.