The DfT's second Road Investment Strategy (RIS2) was published alongside the Budget statement in March.
Projects to be funded under the RIS2 include a Lower Thames Crossing, described as "the biggest roads project of the 21st century"; upgrades to the A303, including a two-mile long tunnel running past Stonehenge; dualling of the A46 past Newark; and a similar upgrade to the A66 Northern Trans-Pennine highway.
In a statement issued today, campaign group the Transport Action Network (TAN) said its lawyers have started proceedings against the strategy.
A claim document lodged with the High Court sets out grounds for seeking a judicial review, including that the transport secretary "unlawfully failed to take account of the impact of RIS2 on achieving specific climate change objectives (when they were a mandatory material consideration)".
It also says the transport secretary "unlawfully failed to take account of the duties placed on him by regulation 17 of the Air Quality Standards Regulations 2010 - namely the requirement to ensure that levels of specified pollutants do not exceed specified limit values, and/or where they do not exceed limit values, to maintain the best air quality compatible with sustainable development".
Chris Todd, TAN's director, said: "Air pollution has breached legal limits for over a decade while greenhouse gas emissions from transport have barely changed since 1990. With 2020 set to be the hottest year on record, we cannot put off urgent action any longer.
"Even when all the evidence points to a need to change direction, the Department for Transport has been unable to kick its addiction to road-building. Only a resounding defeat in the courts can shake it out of its stupor."
TAN said that the DfT is required to respond to the campaigners' arguments within 21 days.
It added that, because of the case's national significance, assuming the High Court grants permission, the legal challenge is expected to be heard by the autumn.
The DfT has been contacted by Planning for comment.
Last week, the government's consent for a new gas-fired plant at the Drax power station in Yorkshire, taken against the recommendation of a planning inspector, was upheld by a High Court judge who ruled that the ministerial approval was in line with national planning policy.
In February, the government's Airports National Policy Statement (ANPS) favouring a third runway at Heathrow was declared unlawful by the Court of Appeal because the UK's obligations to tackle climate change under the Paris Agreement were not taken into account.