A British-born Muslim has admitted plotting to blow up an aircraft bound for America in the first major prosecution of an al Qaeda terrorist in Britain since the September 11 attacks.

Saajid Badat, 25, had planned to set off a bomb at the same time as the British "shoe bomber" Richard Reid, who is serving a life sentence in the United States. But Badat changed his mind and dismantled his shoe bomb, which was seized by police at his home in Gloucester in November 2003.

Anti-terrorist officers believe Badat became radicalised by extremists at a mosque in south London in the late 1990s after he fell out with his father. He then travelled to Afghanistan where he spent about two years at an al Qaeda training camp.

During a 15-minute hearing at the Old Bailey yesterday, he admitted conspiring to blow up an aircraft between January 1999 and November 2003. The guilty plea took the authorities by surprise because he had been expected to stand trial for the offence.

Badat and Reid, who was sentenced to 120 years in the US after being caught trying to detonate a shoe bomb on a flight to Miami, went to the same training camp in Afghanistan, possibly at Khalden. Both men are believed to have received training in suicide bombing and were given explosive devices designed to evade airport security and destroy an aircraft in flight. The bomb found in Badat's bedroom was identical in some respects to the device hidden in Reid's shoe.

Deputy Assistant Commissioner Peter Clarke, the head of the Metropolitan Police's anti-terrorist branch, said: "Today's conviction demonstrates the reality of the threat we are facing. Badat had agreed to blow up a passenger aircraft from Europe to the US and was prepared to kill himself and hundreds of innocent people.

"We must ask how a young British man was transformed from an intelligent, articulate person who was well respected, into a person who has pleaded guilty to one of the most serious crimes that you can think of."

Badat's admission shocked his family and friends in the Muslim community in Gloucester.

The Old Bailey was told that in December 2001 Badat had booked a ticket to fly from Manchester to Amsterdam in preparation for an onward flight to the US on which the explosive device would be detonated. But intelligence sources say no onward flight was booked from Amsterdam.

Instead, the court heard, on December 14 2001, four days after Badat returned to Britain, he emailed his superiors "indicating he might withdraw". It remains a mystery why Badat had second thoughts about the bombing.

Born in Gloucester to a strict Muslim family who moved to England from Malawi, Badat was an active member of his community and had hopes of becoming an Islamic priest. He was educated at Crypt Grammar School for Boys in Gloucester, where he gained four A-levels.

He is understood to have rowed with his father, Mohammed, a retired factory worker, and became increasingly serious about his religion. He spent some time at a mosque in south London, where counter-terrorist sources believe he was infused with radical ideas.

Phone cards found on Reid were said to have been used by Badat to contact Reid's terrorist link, a man named Nizar Trabelsi, who is in jail in Belgium.

After Badat aborted his mission he enrolled in a five-year course at the College of Islamic Knowledge and Guidance in Blackburn, Lancashire.

Badat came under investigation after his name was obtained by MI5 as someone who had spent a suspiciously long time in Afghanistan. He came under intense surveillance in an operation that involved anti-terrorist officers travelling to 15 different countries.

In November 2003, officers raided his family home and arrested Badat. They found an explosive device with detonating cord identical to that used in Reid's bomb.