Tony Blair is to pay any fines his family's people carrier incurs because of London's congestion charge.
The family is eligible for a discount
Mr Blair has been paying a lower level of fee restricted to residents within the charging zone, even though his car has not been registered for the discount.
The prime minister has paid the correct fee but will have been liable for fines if the car has been used in the zone because of his failure to register it.
Downing Street said Mr Blair - currently spending most of his time on the Iraq crisis - had not done this because "he has been busy".
A spokesman said the situation had been "sorted out", adding: "If there are any fines to pay, then he will of course pay them."
Under Ken Livingstone's scheme, the Blairs are entitled to a 90% discount, reducing the charge to 50p a day instead of the usual £5.
Mr Blair has paid the £2.50 weekly charge, but not registered as a resident with Transport for London authority to qualify for the discount.
Failure to register can be deemed a "default", and the car owner fined up to £80 a day for each day the vehicle is driven in the zone.
Mr Blair rarely uses public transport
Downing Street would not respond to speculation the Blairs' use of their Ford people carrier may have incurred fines of up to £1,000.
Opponents of the controversial charge said the prime minister had now had a taste of the inconvenience it had caused for ordinary people.
Vauxhall's Labour MP Kate Hoey told The Mail on Sunday: "I am pleased the prime minister has now seen the pain and annoyance suffered by dozens of my constituents who every day have to face the inflexibility and bureaucracy of this ill-thought out system."
Mr Blair strongly opposed former GLC leader Mr Livingstone's original campaign to be mayor of London, warning that he would be a "disaster" for the capital.
After Mr Livingstone successfully stood as an independent in 2000 against the Labour candidate, former Health Secretary Frank Dobson, he was expelled from the party for five years.