Law firms love to talk about their pro bono exploits – until you ask them precisely what they do. At that point, many grow guarded and suspicious. That was what happened when Legal Week asked firms about their efforts in this area.

The more we investigated, the more it became clear that what is supposed to be a great example of human charity sometimes appears more like a marketing exercise that seeks constant praise but fears any basic scrutiny.

Legal Week's sister title The American Lawyer has been gathering pro bono data from law firms for years. It produces a highly influential annual league table that ranks the firms dedicating the most time to pro bono activities. We decided it would also be interesting to know which U.K. firms put the most effort into pro bono initiatives, so compiled a survey to find out. That's when the whole saga began.

We were told by some firms that comparing the pro bono work they do to that of their rivals was "irresponsible" and "unnecessary". Much of the consternation stemmed from the idea that in the U.K, pro bono thrives not on competition but collaboration, with firms often working together on major projects. So, the theory goes, rankings could damage that collaborative spirit.

To deal with this issue carefully, Legal Week's article praises important collaborative work but does not shy away from basic comparisons, as we felt it would be wrong for firms to use this as an excuse to avoid being transparent on the topic.

Firms that do pro bono in the U.K. are more interested in the outcome of their efforts than the hours put in, firms said, so it would be unhelpful to have a ranking that focused solely on the numbers.

In response, we decided to include factors such as the type of work, the outcome, and the recommendations of peers when ranking the list. We also explained that we were attempting to praise firms that did good work rather than criticise firms that did little. The focus is on the 10 best firms. The ones that do the least pro bono work, according to the survey, are not even included in the article.

But many firms still refused to join in. The largest of these were Linklaters, Allen & Overy, Slaughter and May, Kirkland & Ellis and White & Case – all firms that make a big deal of their pro bono work online. Others were CMS, Bryan Cave Leighton Paisner, Eversheds Sutherland and Norton Rose Fulbright.

Some brazenly said they would wait to see what the coverage looked like this year before deciding whether to take part next year. Others said they did not have the data available – but given that we were asking for very basic numbers such as how many hours are spent on pro bono and how many lawyers do it, plus a few examples, this was either a lie or an embarrassing admission that they don't take the topic very seriously.

Bizarrely, some of these firms actively tried to market their pro bono efforts to us while at the same time saying they would not take part in the survey, which sums up the hypocrisy of those firms desperate for good coverage but fearful of any comparison.

This is not an indictment on the industry as a whole; these firms were in the minority. Most firms we approached took part in the survey and many were able to demonstrate the excellent work they do in this area. Firms such as Hogan Lovells, Ashurst, Herbert Smith Freehills and DLA Piper, for example, clearly take their pro bono work very seriously and there was a feeling that they would be proud of it even if it did not compare well against their rivals.

But it is desperately sad that for the firms that refused, their fear of scrutiny has compromised their ability to disclose basic information on their charitable endeavours.