The security operation at this week's G20 summit was thrown into chaos last night when it emerged that the entire network of central London's wireless CCTV cameras will have to be turned off because of a legal ruling.

The Department for Transport (DfT) has ruled that Westminster council's mobile road cameras - a third of the authority's CCTV network - "do not fully meet the resolution standards required" and must be switched off by midnight tomorrow.

The blackout begins on the eve of the summit, when world leaders arrive in the capital and protesters take to the streets.

The council only discovered last week that images from its newly installed £15m traffic cameras do not meet the quality required under the Traffic Management Act, which comes into force on 1 April.

In an urgently drafted letter seen by the Guardian and hand-delivered to the transport secretary, Geoff Hoon, on Friday, the council warns its entire network of wireless cameras will need to be shut down unless the minister finds a way to give special dispensation. "This would have a serious impact on our ability to manage our road network safely, as well as impeding our community protection efforts," the letter states.

It adds: "We are seeking authorisation from DfT as a matter of urgency to enable Westminster to continue using its digital CCTV network."

The 60 cameras in question use the latest digital technology and transmit images using Wi-Fi. While they are primarily for traffic enforcement, according to the council the cameras are "an essential additional tool" to tackle crime and disorder, and have been fixed to strategic locations across the capital ahead of the summit.

The 24-hour live footage from the cameras, which monitor roads around the West End, Belgravia, Trafalgar Square, Knightsbridge, Oxford Street and London's main bridges, is also accessible to police and the intelligence services.

A further 160 "permanent" CCTV cameras run by the authority are unaffected. However, security officials believe a shutdown of the mobile road cameras could hamper the G20 security operation, which will require police to secure the safe passage of dozens of motorcades carrying delegations VIP diplomats and leaders.

"Frankly, it couldn't have come at a worse time," a source said. "These are not just parking enforcement cameras, they're for public order and we've got the G20 world leaders coming. This is a complete disaster."

Under the legislation, traffic cameras must be capable of recording at 720 x 576 pixels, an analogue broadcast standard.

Westminster's wireless network of road cameras, introduced last year, is the only fully digital traffic enforcement system operating in the UK, and is regarded as one of the most advanced in the world. But its picture quality is only 704 x 576 pixels. The DfT's enforcement branch, the Vehicle Certification Agency, has ruled it does not comply with the law.

DfT lawyers were last night frantically exploring a way to exempt Westminster from the legislation. A department statement said: "To ensure local authorities have a fair and transparent way of detecting unlawful drivers, any recording device must meet minimum requirements."

The council last night confirmed that - barring an 11th-hour U-turn - it would have to switch off the network tomorrow to comply with the act.

Danny Chalkley, the council's cabinet member for environment and transport, said in a statement: "The DfT's position is ludicrous and stuck in the dark ages.

"[This] could result in millions of pounds of taxpayers' money being needed to replace the current cameras, all because of a tiny difference in image resolution on the TV screen."