Image caption Sue Gray, appearing before the Public Administration Select Committee, in 2012

Recently, I have been rooting about for information about the work of the most powerful civil servant you've never heard of. Unfortunately, it turns out that she is also perhaps the most secretive you could ever hope to meet.

Sue Gray works in the Cabinet Office, where she has the asking-for-trouble job title of "Director-General, Propriety and Ethics Team". Her job is to adjudicate on whether rules have been broken by officials, ministers and special advisers. In practice, she is also a fixer for Sir Jeremy Heywood, head of the civil service. She performed much the same role for Lord O'Donnell, his predecessor.

Ms Gray's influence is astounding. She ranges widely: part of her job is signing off memoirs (including by elected politicians) to check they have not revealed anything unhelpful. Officials from right across government have stories of her intervening on everything from their pay and conditions through to what documents they may or may not publish.

Despite being one of the top handful of civil servants, her role is often overlooked and her actions are attributed to Sir Jeremy himself. That happened, notably, during the official investigations that led to the resignations of Andrew Mitchell, then chief whip, and Liam Fox, then defence secretary. She is his right hand.

Officials see her as a force of conservatism within the civil service. She is an internal critic of attempts to open up the old Whitehall machine to competition or scrutiny. But she is, when the circumstances merit it, flexible. Sir Jeremy has described Ms Gray as "user-friendly"; she likes to give advice which ministers like to hear.

Whitehall has powerful officials - that is the way of the world. But I am troubled by the unique secrecy - even by the standards of our unaccountable civil service - of the leadership of the Cabinet Office, a department whose decisions ripple through officialdom and, from there, out into the real world.

Leaving no trail

First, Ms Gray is notorious (like Sir Jeremy) for her determination not to leave a document trail.

When Labour wants to propose someone for a peerage, they ring her on the phone. She then gives them an oral "yes" or "no" as to whether they are likely to get through vetting. The party accepts her verdicts, but there are no records and she never explains herself. There is simply no check on what basis she performs the first sift on House of Lords membership.

I have written before about my attempts to get advice to special advisers about what work they may do for parties while they are public employees. There are, after all, strict rules designed to reassure the public that special advisers work for the taxpayer, not their parties. But some special advisers have broken these rules by campaigning and canvassing - seemingly with official consent.

So Stephen Parkinson, a special adviser at the Home Office, wrote to her for confirmation of the rules. This put Ms Gray in a bind: replying would reveal that either she had wrongly failed to enforce the code against other advisers or she had - in secret - abolished the code. Neither would be helpful to ministers. So she did not answer. On seven occasions.

As a consequence, Mr Parkinson (and Nick Timothy, then his colleague) refused to campaign and they stuck by the rules. They are now barred from being Tory parliamentary candidates for having failed to do their part for the party. They are paying a price for Ms Gray's apparent refusal to make a ruling on a core "ethics and propriety" issue.

Even when a document trail exists, Ms Gray is enthusiastic about keeping it a secret. We know that she advised special advisers on how to destroy email (by "double-deletion") to thwart FOIA requesters. She did not advise them on the requirements of the code on public record-keeping. We also know, via the Freedom of Information Act, that she kept no log of why, how or when she destroys documents (contrary to that guidance).

Letting no sunshine in

It goes on: I know of half-a-dozen occasions where Ms Gray has intervened to tell departments to fight disclosures under the Freedom of Information Act. Civil servants at the Department of Health (DH) state that she was behind that department's doomed attempts to keep the diary of Andrew Lansley a secret - a case that, because there was no plausible case for secrecy, collapsed into farce.

In that case, someone wanted (quite reasonably) to see what range of people had met and lobbied Mr Lansley during the preparation of the Health and Social Care Act of 2012. The DH officials, struggling for an argument, claimed that if diaries were allowed to be published, ministers would schedule pointless meetings so that members of the public would not think they were lazy.

The judge ruled that the evidence and argument fell "way below the standards that the public and the [court] are entitled to expect of government departments and senior civil servants in advancing public interest arguments".

Sometimes, however, Ms Gray gets caught out. The Cabinet Office has, in fact, just been forced to release some of her advice.

A chink of light?

In September 2011, Ms Gray advised Michael Gove, then education secretary, that email sent in private email accounts on government business was exempt from transparency laws. He had been using his wife's email account for departmental business. Ms Gray did so over the telephone (of course), but also followed up with an explanatory email.

If data is not stored on servers accessible to officials, she wrote, it "seems obvious that they cannot 'hold' it for the purpose of the Act", she wrote. But government business is government business. Mr Gove's use of his wife's email account (the so-called "Mrs Blurt" account) when he emailed officials created no exemptions. This duff advice ended up costing tens of thousands of pounds in legal fees.

When I first requested a copy of this email back in 2011, the Cabinet Office insisted that it did not exist. The advice had only been given over the telephone, I was told. It was only in the course of a legal case (involving me, when I was a reporter at the Financial Times) in 2012 that the Department for Education revealed that it existed and it had kept a copy. The DfE revealed that the Cabinet Office had told an untruth.

Why and how did this happen? I do not know. I've asked the Cabinet Office about this - and the rest - but they are loath to comment on it.

Recently, chasing a lead, I asked for emails sent by and received by Ms Gray in a specific 12-hour window which relates to government work. This request was deemed "vexatious": I am being a pest, you see. What possible benefit could arise from seeing what she did in that half-a-day?

Given the range and scale of her quiet influence, I suspect any 12-hour period would reveal a lot. If, indeed, we found any records at all.