Help is at hand for drivers who only know when they have entered London's congestion charging zone after a penalty notice lands on their doormat.

Research suggests that up to 4,500 people a day are fined for not paying the congestion charge when they drive through central London.

After being caught out himself, inventor Matthew McCluskey, has created a gadget that keeps an eye on a vehicle's location.

When it spots that a driver is about to enter the congestion zone it automatically triggers a payment.

Cost cutting

The idea for the gadget, dubbed the KenBuster, was born 18 months ago, when Mr McCluskey returned from holiday to find a large fine waiting for him.

"I had driven to the airport, and of course because I was looking forward to my holiday, I was so excited, that I forgot to pay the charge," he told the BBC News website. "I realised quite quickly that there had to be a simpler way of doing this."

The KenBuster uses a GPS unit to track the car's location and then uses GPRS mobile phone technology to despatch payment details.

The gadget, registered to a specific vehicle, is a small box about the size of a mobile phone that is charged by being plugged into a car's cigarette lighter socket.

Mr McCluskey said he hoped it would make life easier for drivers.

"There is definitely a hassle free factor to it," he said. "It is quite clear that people find it a pain to pay over the internet or go into a shop."

According to information from 2005/6 accounts, Transport For London (TFL), which administers the congestion charging scheme, made a total revenue of £210m. Of that total, £65m, came from congestion charge fines.

Approximately 4,500 people are fined per day - roughly 6% of those who drive in the congestion zone.

Penalties for not paying the charge can high. The basic penalty fee is £100 but this is reduced to £50 if it is paid within two weeks of being issued. The penalty increases to £150 if it is not paid by 28 days after the notice was first issued.

Persistent non-payers risk having their vehicle clamped or confiscated.