A group of MPs and environmental campaigners have condemned the "cover-up culture" at Highways England amid a row about plans to build a dual carriageway through a country park.
Bill Esterson, the MP for Sefton Central, said his office obtained the emails after he submitted a Freedom Of Information request for the minutes of meetings between private companies partnered with the A5036 Port of Liverpool access scheme.
In the emails, Highways England officials described similar requests made numerous times by campaigners, including Rimrose Valley Friends (RVF), as "manifestly unreasonable" and "vexatious", and that they represented a "concerted campaign to disrupt the work of the authority".
It is the latest development in a long-running campaign against plans to build the road through Rimrose Valley, between Crosby and Litherland, in order to provide more access to the Port of Liverpool for lorries.
The scheme aims to build the new route to connect the port with the motorway network bypassing the A5036.
One official on the scheme said a reason for continuing to resist the FOI request "is the damaging effect that releasing the records is likely to have on our relationships with stakeholders".
The email goes on to say that if campaigners made an appeal to the Information Commissioner's Office (ICO) and if it ruled in Highways England's favour, then "it could go a long way to closing down the FOI/EIR (environmental information regulations) route for RVF in future".
Highways England said it has "acted appropriately" to the FOI requests and that the project team is open to discuss concerns about its plans with interested parties.
Mr Esterson's FOI request was rejected despite Highways England telling him in a letter that they "do hold information that is relevant to your request".
Mr Esterson said: "These emails show an outrageously arrogant attempt by Highways England to cover up a legitimate request for information through the Freedom of Information Act 2000.
"In merely carrying out our duty as a Member of Parliament, representing our constituents, we were met with obstacles to transparency at every turn.
"It speaks to an insidious cover-up culture at Highways England which must be investigated."
The Labour MP for Bootle Peter Dowd, who also opposes the scheme, added: "Highways England say they aim to 'ensure their activities result in a long term and sustainable benefit to the environment'.
"Rather than actively trying to obstruct Bill Esterson MP and me in representing our constituents, they should act on their own aims by bringing a halt to this proposal which, amongst other things, will strip out the only significant green space in my constituency."
Countryside charity CPRE, The Wildlife Trusts and campaign groups Transport Action Network and Save Rimrose Valley support the MPs.
Craig Bennett, chief executive of The Wildlife Trusts, said: "It seems extraordinary that a huge dual-carriageway right along the length of the beautiful Rimrose Valley could even be considered.
"The proposals are an appalling affront on local communities, on nature, and any sort of suggestion that our society might try to 'build back better' after the pandemic crisis."
A Highways England spokesperson said: "Highways England takes its legal and ethical responsibilities under the Freedom of Information legislation very seriously and is fully committed to the transparency agenda and meeting all the requirements of the FOI Act.
"As part of that commitment we are working towards automatic publication of all FOI responses and exploring methods of publishing more of our data."