Cabinet Secretary Sir Jeremy Heywood's department has the worst record for transparency in Whitehall, with nearly six in 10 information requests denied, it was reported.
The man whose office is running a cross-party review of the Freedom of Information (FoI) Act, has previously claimed it has a "chilling effect" on the effective running of government.
However it is feared any proposals may seek to water down the laws that facilitate the release of information, particularly facts that are damaging to those in power.
An investigation by the Daily Mail revealed the Cabinet Office, which handles enquires made to Downing Street, rejected 57% of requests, compared to the average of 34% across all Whitehall departments.
The department with the best record for transparency, UK export and finance, denied requests for only 6% of cases.
Liberal Democrat leader Tim Farron told the newspaper: "Accountability and transparency are words that regularly come out of the mouth of the Prime Minister but these figures show that these words are hollow.
"Unlike the Prime Minister, I still believe that sunshine is the best disinfectant.
"The Cabinet Office should be leading by example, not trailing behind miserably."
The review, chaired by Labour deputy leader Tom Watson, is said to be looking at changes that will undermine the Act, such as giving officials and ministers increased powers of veto and imposing charges on requests.
The figures, themselves obtained by exercising FoI, showed that between July and September last year the Cabinet Office received 497 requests under the Act.
In 290 of the cases the information was held, however just 55 were approved in full, compared to the 165, or 57%, that were "fully withheld".
In 39 of the cases some of the requested information was withheld, while 31 were yet to see a decision.
A Cabinet Office spokesman told the Daily Mail: "The Cabinet Office is absolutely committed to Freedom of Information, and we have led the Government's drive to publish more data more regularly than ever before.
'We are working hard to make our processes even more efficient."