Gridlock as 800 traffic lights seize

Central London was brought to a standstill in the rush hour when 800 sets of traffic lights failed at the same time - in effect locking signals on red.

Thousands of drivers were trapped between the north and south circulars as vast tailbacks grew. Tempers frayed with most major routes across central London impassable.

The worst gridlock the capital has seen for years was caused by a computer which crashed as engineers installed software designed to give pedestrians longer to cross the roads.

Cherie Blair was among those caught in the chaos as drivers were forced to switch off their engines and wait. Many abandoned cars to sit by the roadside as the chaos caused by the traffic light failure was made worse by the largest programme of roadworks the capital has ever seen.

Mrs Blair reportedly left her car on the way to an important engagement and caught the Tube at Oxford Circus.

The worst-hit routes centred on Trafalgar Square, already the scene of huge lane closures. The chaos worsened when up to 30 buses became trapped by the congestion and blocked off escape routes to all other traffic.

"These are the worst conditions we have seen for a long time with motorists completely blocked in," said Rebecca Rees of the AA. Thousands of traffic lights are being re-phased as part of Mayor Ken Livingstone's Transport for London (TfL) plan to give pedestrians longer to cross roads, to redirect traffic away from sensitive sites and to speed bus trips.

Critics claim that the re-phasing will be reversed when congestion charging starts, to give the impression that the £5-a-day scheme is improving traffic flow.

The computer failed at 6.15am. TfL said engineers were immediately scrambled to fix the software problem.

But it took until 8.30 to get the computer running again. Traffic was still at a virtual standstill well after 10am. At midday Mr Livingstone said he could not comment on the chaos because he was unaware of the situation.

When the computer crashed, hundreds of traffic lights instantly switched to "local" phasing patterns instead of working in a complex, coordinated-scheme intended to keep traffic-flowing. At the height of the chaos TfL attempted to improve the traffic flow by rushing staff to Trafalgar Square in brightly-coloured vests to prevent people from parking illegally.

"We wanted to stop goods vehicles and so on from stopping at the side of the road and making conditions worse," said a TfL spokesman.

A TfL spokesman admitted that the crash was caused when engineers attempted to install a new computer "model" - complex software linked to controversial plans aimed at re-phasing traffic lights to give pedestrians more precedence over motorists.

They said the computer unexpectedly "choked" on the new software.

Motorists heading along Whitehall towards Trafalgar Square were among those worst affected. Elsewhere, the AA reported six-mile tailbacks on the M4 into London after a vehicle broke down on the elevated section at junction two.