John Howell QC, for the campaigners, said the Government's unwillingness to give details of a deal with the private company to build the Birmingham Northern Relief Road was "flawed and unlawful".
He accused John Prescott, Secretary of State for Environment, Transport and the Regions, of acting in breach of his statutory duty to allow the public access to information.
Labour once opposed the scheme. But when in power, the Government said last July that the 27-mile road, to be built by a private company, Midland Expressway and paid for by tolls, should go ahead. Mr Howell, for the Alliance Against the Birmingham Northern Relief Road, yesterday asked the High Court in London to force the Government to be more open by quashing a refusal to reveal information from the agreement, signed in 1992, excluding those parts covered by commercial confidentiality.
John Prescott last year said he would not disclose any part of it.
But Mr Howell said that under European Union directives allowing freedom of access to information on the environment, the Government could not keep the whole agreement secret just because it might contain "some information" which was commercially sensitive.
The rest of the information "must be disclosed unless it is incapable of being separated from any information which is protected".