A protest is being held at the Dartford Crossing on Friday by campaigners who claim the tolls should be scrapped.
There is a flat rate of £1 for car drivers who use the crossing
The UK Independence Party (UKIP) says charges breach European laws because the cost for trucks is too high.
UKIP MEP Nigel Farage said the government promised to scrap the charges once the crossing was paid for, which happened in 2003.
But Transport Minister Steve Ladyman insisted the tolls control the flow of traffic so must stay to ensure safety.
"One of the purposes of tolling is that cars are effectively metered as they cross the bridge so it does not exceed its design capacity - it's a safety thing and that's perfectly legal," he said.
"We had a public consultation in 2000 about what we should do about the tolls and the public came to the conclusion the best thing was to continue the toll as a congestion charge."
But Mr Farage insisted the tolls are illegal under European law because lorries are charged too much money at peak times compared with other times of day.
Cars are charged a flat rate of £1 to use the crossing but lorries have to pay £2.90 during the day, which is reduced to £1 at night.
Mr Farage said the government had also broken the public's trust "by turning its back on a promise".
He is one of three UKIP MEPs, along with representatives from the Freight Transport Association, staging a protest at junction 1 of the M25 in Kent.
The crossing, which forms a vital link to the London orbital road, comprises two tunnels carrying traffic to the north, and a bridge carrying traffic to the south.
Last year it was used by 54,363,607 vehicles.