The Department for Transport (DfT) announced its preferred corridor route for the western section of the proposed expressway linking the two university cities in September.
The expressway is a key element, along with an East West Rail link, of government-backed plans put forward by advisory body the National Infrastructure Commission (NIC) for up to a million new homes in the Oxford-Cambridge arc by 2050.
As recommended by government agency Highways England, the selected "Route B" option starts from the M1-A421 junction in Bedfordshire, runs south of Milton Keynes, across north Buckinghamshire and into Oxfordshire, with options to run east or west of Oxford.
In the letter, the 11 MEPs, from the Liberal Democrat, Labour and Green parties, said: "We want to express our deep concern about the proposed 'Corridor B' on the grounds that no strategic environmental assessment (SEA) or habitats regulation assessment (HRA) has been undertaken as is required under European Law.
"The UK government has been carrying out SEAs and HRAs on the environmental impact of many proposed major developments in the UK since their introduction in 2001. However, there has been no such public consultation on the highly controversial expressway, which will lead to the development of up to one million homes on or near some of the richest biodiversity nature reserves in the country like Otmoor Basin and Bernwood Forest."
The letter goes on to say: "Ploughing ahead with this expressway project, without first conducting an open and thorough public consultation, sends very worrying signals about whether, despite the commitments it has made, the government will maintain European-wide environmental standards in practice [after Brexit]."
The letter also refers to a legal challenge that has been launched by the Berks, Bucks and Oxon Wildlife Trust (BBOWT) and the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire (BCNWT) against the DfT and Highways England for failing to undertake and SEA or HRA.
The trusts announced that they had launched the challenge at the end of September with support from the Environmental Law Foundation. Leigh Day Solicitors and barristers from Francis Taylor Building were providing pro-bono advice it added.
Tessa Gregory, partner at Leigh Day, said at the time: "It is important that decision-makers listen to local organisations and local people and imperative that environmental effects are properly assessed at the appropriate stage.
"Corridor selection is precisely the time for such strategic assessment to be carried out so that the environmental impacts of a plan or programme, such as the Expressway Corridor, are understood.
"It is disappointing that the secretary of state has pressed ahead without such assessment, despite consultation submissions from the Wildlife Trusts."
A DfT spokeswoman said: "The Oxford to Cambridge Expressway will improve transport connectivity and growth across the region, and the benefits will be felt by the whole country.
"Protecting the environment is central to how it is developed – we have been engaging with community groups about developing ideas for a road in this area since 2015, and will continue to do so leading to a consultation on specific route proposals next year."
She added that the environment was "a key consideration for selecting a corridor" and the department will undertake a full environmental assessment on the final route.
An article analysing the impact of the road corridor announcement on planning in the arc can be found here.
A feature examining local authorities' response to the government's plans for the Oxbridge corridor can be found here.
The government announced last month that it would be continuing with the current environmental regulations regime after the UK leaves the European Union next March.
*NOTE: this story was updated at 4pm on Wed 7 Nov to add a comment from the DfT