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.
The report says that, despite carrying 50 million vehicles a year, the status meant nitrogen dioxide recordings were not reported to the EU.
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) - told the BBC, "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."
It says she added that the Department for Transport (DfT), which is responsible for road classification, confirmed the rural status "was incorrect", but that the DfT told the BBC it was Defra that designated the A282 as a rural road.
It has now been reclassified.
