Only one in 6,000 motorists who use their mobile phone while driving in London are caught and fined, according to figures released by the Lib Dems.
Laws were toughened this year
The study showed 50 fixed-penalty notices were handed out every day despite 2.6% of people being seen using handheld phones while driving.
Drivers in Hammersmith and Fulham, west London, were the worst offenders with one in 10 breaking the law.
A ban on using handheld mobile phones while driving came into force in 2003.
In February laws were toughened so offenders would face a fixed-penalty fine of £60 and three penalty points on their licence.
Between February and May the Metropolitan Police issued 7959 fixed penalty notices for the offence compared with 7008 for the same period last year.
Lib Dem Shadow Transport Secretary Alistair Carmichael MP said: "Driving and using a handheld mobile phone at the same time is recklessly dangerous to other road users and pedestrians.
PLACES MOST DRIVERS SEEN USING A MOBILE PHONE
Hammersmith and Fulham
"It is deeply concerning that so few drivers doing this are ever caught."
He said a drop in the number of traffic police in the Met was partly to blame.
"You simply can't catch people using their mobile phones whilst driving if traffic police are replaced with speed cameras.
"Rather than wasting billions on ID cards the Government should invest in more police who can properly enforce road safety rules."
The Met said a number of enforcement campaigns were taking place in conjunction with with Transport for London (TfL) this year.
TfL commissioned the report to survey the use of mobile phones by car and van drivers at a range of sites in London.
The research was carried out for 43 days, from March 2006, at 33 sites across the city, one site in each borough and one in the City.
According to the study drivers in Kensington and Chelsea were most likely to receive a fine and those in Hillingdon were most likely to escape a penalty.