The government has failed to include the Dartford Crossing in assessments of air quality pollution because it classed it as a rural road.
The crossing is the A282 and carries 160,000 vehicles daily - even though its capacity is for 135,000.
The blunder could have meant the government has avoided fines for breaching EU pollution limits.
Dartford council says it became aware of the issue when it realised the road had been excluded from the government’s National Air Quality plan.
Officers were concerned because their own measurements showed the crossing exceeded the EU target for nitrogen dioxide.
Cllr Keith Kelly, the council’s cabinet member for transport, said he was shocked and the revelation underlined the importance of building the third Thames crossing.
“There has been a downward trend [in pollution levels] but recently it has been coming back up again. Our environmental team noticed there was a discrepancy in the figures for the national Air Quality Plan.”
“We queried the figures and it turned out they [the government] had classified it as a rural road. It was shocking to discover that.
"It seems a genuine oversight but how - I don’t know. It was hugely disappointing and we have only heard in the last few weeks from Defra that it was the wrong classification.”
It meant that fines that could have been imposed by the EU were not and the knock-on impact when there were problems with the crossing was under-estimated.
Cllr Kelly said: “Around the A282 there are more than 700 homes and 1,500 residents and those people are breathing in quite dangerous levels of pollution, particularly when there is stationery or slow moving traffic.”
“What we need is to get traffic away from Dartford and the quicker the better. The sooner we get the decision on the third crossing, the better. It can’t come quick enough for the residents of Dartford.”
There was also criticism from the Green MEP Keith Taylor. He said: "The misclassification of the Dartford Crossing as a rural road is, at best, outrageous incompetence, at worst, a dangerous attempt to avoid taking action on a deadly air pollution crisis that unnecessarily claims the lives of more than 700 people in Kent."
"That neither the DfT nor Defra corrected the error, despite having many an opportunity to do so, further demonstrates the government's extremely concerning indifference towards its duty to safeguard the health of citizens in Kent."
The Department for Transport has acknowledged the error and says data will now be included when reporting assessments to the EU.