The US embassy is the biggest charge evader
The cost of embassies evading the congestion charge in central London has soared 40% in a year - with the sum now equivalent to £5 for every Londoner.
In 2009 the unpaid bill for embassy staff who refuse to pay the £8 charge then £120 fine was £36m. Diplomats have immunity so cannot be taken to court.
Liberal Democrat Assembly Member Caroline Pidgeon said it was "shameful" that embassies evade the charge.
Transport for London pointed out that 74% of embassies now regularly pay.
But Ms Pidgeon said: "British diplomats respect the law of other countries and it is only right that diplomats do the same in this country.
"The congestion charge is exactly what it says on the tin. It is a charge for driving a vehicle at certain times of the day within highly congested parts of London.
"It is no more a tax than road tolls in France or tolls to cross bridges in the US."
'Not a tax'
At the moment the unpaid bill is picked up by Transport for London (TfL) and the Mayor of London.
That means the cost is effectively handed to the public, Ms Pidgeon warned.
She said: "The £40m owed by embassies is money being denied to all Londoners.
"By not paying the congestion charge Londoners are having to pick up the bill, either through higher fares or investment being denied in much needed transport projects."
A TfL spokesman said: "TfL and the UK Government are agreed that the congestion charge is a charge for a service and not a tax which means that diplomats are not exempt from payment.
"TfL has held discussions with a number of embassies about payment of the congestion charge. As a result of these discussions, a number of previously non-paying embassies are now paying the charge."