US diplomatic staff will be regarded as "shockingly mean" for refusing to pay the congestion charge, London transport commissioner Peter Hendy has said.
The embassy owes £270,000 in unpaid charges
Staff at the American embassy have run up a bill of £270,000 in the past six months, saying the £8 charge is a tax they should not have to pay.
But Mr Hendy insists they must, saying other motorists and Londoners would resent any special treatment.
Last month, mayor Ken Livingstone said the US could afford to respect UK law.
Speaking on BBC Radio 4's Today programme, Mr Hendy said: "There's a direct benefit from paying the charge which is that if you drive into the congestion zone, you get a faster, less congested journey.
"You don't have to pay it. You can walk, you can cycle, you can travel by public transport."
Deputy chief of mission for the US embassy in London David Johnson said it was a tax, meaning diplomats were exempt.
"We have agreed for a long time that it is inappropriate for one government to pay taxes to another, " he said.
He added that he paid his taxes to the United States government, even though he lived in the UK.
"That is the appropriate thing for me to do and the appropriate thing for your diplomats to do as well."
But Mr Hendy said: "I think a lot of people who live and work in London will regard what has just been said as shockingly mean from the richest country in the world. They should pay up now."