Pollution levels around the Dartford Crossing have been excluded from government air quality assessments because it was classed as a "rural" road, the BBC has discovered.
Despite carrying 50 million vehicles a year, the status meant nitrogen dioxide recordings were not reported to the EU.
Consisting of the Dartford Tunnel and QEII Bridge, the crossing - officially known as the A282 - connects the M25 north and south of the River Thames.
It has now been reclassified.
In a letter obtained by the BBC, government minister Therese Coffey conceded the error.
Dr Coffey - responsible for improving air quality on behalf of the Department for Environment, Food & Rural Affairs (Defra) - said: "The A282 in Dartford does not appear in the national air quality plan for nitrogen dioxide because it was classified as rural and was, therefore, excluded from Defra's air quality modelling assessment."
She added that the Department for Transport (DfT), which is responsible for road classification, confirmed the rural status "was incorrect".
However, the DfT told the BBC it was Defra that designated the A282 as a rural road.
The error was only recognised because Dartford Borough Council noticed the stretch of road was not included in the government's National Air Quality plan.
For 15 years the council has carried out its own air quality measurements, and each year the area around the crossing has been above the EU's target for nitrogen dioxide.
It said it passed the data to Defra, but no action was taken.
Councillor Keith Kelly, the council's head of transport and infrastructure, said the revelation was "shocking" as for years key pollution data was not seeing the light of day.
He added he was "hugely concerned" about the state of people's health because, despite the crossing being labelled as an A road, the eight lane dual-carriageway was effectively a "motorway running through the middle of our town".
The road, according to Highways England, is routinely "full to capacity".
It is "one of the least reliable sections of the UK's road network" and "congestion at the crossing quickly backs up to affect local roads".
Public Health England has estimated Dartford has one of the highest percentage of deaths that can be attributed to long-term exposure to particulate air pollution in Kent.
Particulates are the deadliest form of air pollution due to their ability to penetrate deep into the lungs and blood streams unfiltered.
Thurrock, at the northern end of the crossing, has the highest estimated percentage in the East of England.
Defra has now promised to include the data "in any future assessments reported to the EU".
You can see the full story on Inside Out, on BBC One South East and on Monday 6 March at 19:30 GMT, and later on the BBC iPlayer.