According to latest figures released by Transport for London (TFL), India owes the department 3,323,400 pounds in Congestion Charge dues from February 2003 to January 2014.
"We and the UK government are clear that the Congestion Charge is a charge for a service and not a tax. This means that diplomats are not exempt from paying it," TFL said in a statement.
The US leads the list of embassies refusing to pay the 10-pound levy on vehicles entering a marked zone in central London during peak hour traffic on weekdays.
The total in unpaid bills from over 100 embassies and high commissions in London adds up to nearly 77 million pounds.
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"Around three quarters of embassies in London do pay the charge, but there remains a stubborn minority who refuse to do so, despite our representations through diplomatic channels," TFL said.
"We will continue to pursue all unpaid Congestion Charge fees and related penalty charge notices and are pushing for the matter to be taken up at the International Court of Justice."
According to the UK Foreign Office, the Congestion Charge is considered a "service rendered" under diplomatic rules, though legal immunity means diplomats cannot be prosecuted for non-payment.
Foreign Secretary William Hague revealed last year that officials had met foreign missions and asked them to pay outstanding fines or appeal against them.
Besides the India and US missions, the top 10 defaulters include the High Commission of Nigeria, the Embassy of Poland, and the Sudanese Embassy.