Whitehall won't say if papers withheld from Leave ministers
The government has refused to say how many documents have been withheld from ministers who back leaving the EU.
Under special referendum rules pro-Leave ministers cannot use the civil service to bolster their case.
But the Cabinet Office rejected an FOI request asking for details of how the rules were applied across Whitehall.
Two departments - energy and defence - said no papers had been restricted from their Brexit-backing ministers and four others refused to give any details.
The four departments which were headed at the start of the campaign by secretary of states backing a departure from the EU were the Department for Work and Pensions, the Northern Ireland Office, the Ministry of Justice and the Department for Culture, Media and Sport.
The government is campaigning for a vote to stay in the EU, but "exceptional arrangements" have been put in place allowing ministers to campaign for either side.
'Unelected and unaccountable'
As part of the temporary rules, restrictions were put in place on the official documents that would be made available to ministers campaigning for the UK to leave the EU.
The guidance, set out in February by Cabinet secretary Sir Jeremy Heywood, was criticised by Leave campaigners, with one minister calling the curbs "unconstitutional".
Sir Jeremy sought to reassure MPs, saying the ban only applies to "briefing and speech material supporting the Out position".
The Commons Liaison Committee recently said a list of withheld papers should be published to give the public more confidence in the process.
Its chairman, Conservative MP Andrew Tyrie, wrote to David Cameron warning the guidance appears to pass ministerial responsibility for a department to "unelected and unaccountable civil servants".
In response to Mr Tyrie, the PM said he did not think it necessary to produce a list, adding: "I can reassure you that we are not aware of any difficulties caused by the current guidance and would not expect it to interfere with any minister's statutory or constitutional responsibilities."
Mr Tyrie replied, saying: "This is an unsatisfactory arrangement and could turn out to be unsustainable."
He added: "The prime minister - and the public - would benefit from a commitment to publish a list of the official departmental papers which will not be made available to pro-Brexit ministers."
In responding to a Freedom of Information Act request from the BBC, the Cabinet Office refused not only to publish a list of papers as requested by Mr Tyrie, but would also not say how many times documents had been withheld and which ministers or departments it had applied to.
It said it did hold the relevant information and recognised the "specific public interest" in giving more details of the arrangement.
But it said to do so would weaken ministerial accountability as well as "damage the perception of the neutrality of the civil servants working in those offices and undermine future working relationships".
Separate requests to individual departments were refused using a section of the Freedom of Information Act aimed at protecting "the formulation of policy, communications between ministers, and the operation of ministerial private offices".