The vehicle sped off from police near London Bridge and was later found abandoned.

After a controlled explosion, empty fertiliser bags and architects' drawings of City buildings were reportedly found in the car, which had Northern Ireland number plates.

Abandoned car

Fertiliser car bombs exploded outside the BBC Television Centre in Shepherd's Bush, west London, in March, 2001, and in Ealing, west London, in August.

And senior anti-terrorism detectives have spoken of the possibility of a mainland attack by dissident Irish republican groups.

But a City of London spokeswoman told BBC News Online it was too early to consider that the Queen Mother's funeral day may have been targeted.

She said: "We cannot even say whether this is linked to Irish terrorism - it's still too early to say.

"There wasn't a bomb in the car, but it's an avenue we're looking at and we're in a heightened state of awareness.

"Indications are suspicious, which is why we need information on the vehicle."

At 2000BST on Tuesday, a police officer stopped a car near a security point at London Bridge and while attempting to speak to the driver, it sped off.

The car was traced using CCTV and later found abandoned in nearby Mincing Lane, north of the Thames, minus its driver and passenger.

Police sealed off the area around the blue-grey Ford Granada and carried out a controlled explosion.

Detective Chief Inspector Steve Eastwood of City of London Police has appealed for the occupants of the vehicle or anyone who saw the car that night, to come forward.

Previous attacks

The registration number was IDZ 4669 and the driver was white, probably in his late twenties, sitting next to a white woman with tied back light brown or fair hair.

BBC home affairs correspondent Danny Shaw said: "It might not be connected to terrorism at all.

"But clearly when you have got this combination of events - Northern Ireland number plates, a car abandoned in the City of London that didn't stop at the ring of steel, architects drawings of City buildings and the fertiliser bags -it's obvious the police are keen to find an explanation for it."

The last suspected Real IRA attack on the mainland was in Birmingham in November last year.

Dissident Irish republicans were also accused of trying to wreck the Queen Mother's birthday pageant in 2000 after an explosive device was found at Ealing Broadway station in west London.

Information about this incident to the Anti-Terrorist Squad Hotline on 0800 789321.