The recent profitability tables of the top 50 UK-based law firms ( Legal Week 20 July) demonstrate that the fragmentation of the UK market which became evident a couple of years ago is now established.

The figures must be viewed with some caution. Figures in any one year may be distorted by special factors, including a premises move or the cost of partner de-equitisation. As an increasing number of firms become limited liability partnerships, audited figures will become available using similar accounting conventions, thereby enabling effective comparison between rival firms. In the meantime, variations between firms in profit per equity partner and revenue per fee earner of up to 5% can probably be discounted as 'rounding differences'.

Our first analysis was to look at profit per equity partner. Any firm can rapidly increase its profits if it starves investment in recruitment, training and IT so a sustainable trend of improving profit per equity partner (PEP) is far more relevant as to a firm's position in the market.

We identified four clear tiers of firms based primarily on PEP.

Magic circle

Although firms in this elite group are not always the most profitable, they have developed an enviable position in the market. They have a breadth and depth of practice and reputation that, subject to client conflicts, almost gives this group a right of pre-emption to be involved in any major corporate or financing transaction.

There are, however, interesting variations between the firms. Slaughter and May is by far the smallest magic circle firm and primarily operates internation-ally via top-ranked 'best friends'.

This is the Cravath Swaine & Moore of London. It remains highly profitable but faces an increasing challenge in the international deal league tables.

Allen & Overy – traditionally one of the most profitable magic circle firms – put in a slightly disappointing performance. This may be due to its under-weight corporate practice, compared with the rest of the magic circle, or it could be connected to its forthcoming London office move.

The stars

These firms are generally as or more profitable than the magic circle. They have especially strong practices, often in corporate. However, they probably lack the geographic and/or practice depth or breadth of the magic circle. These firms have performed strongly and, with the magic circle, have placed clear blue water between themselves and the rest of the UK market. Together with the magic circle, they constitute the premier league of UK firms. Indeed, it is likely that the strengthening London and European practices of the major US firms are – and will be – far more potent competition for the premier league firms in the UK than the firms currently occupying the lower tiers.

The inbetweenies

These firms have produced good earnings growth and are now generating profits of not less than £550,000 per equity partner. However, this tier is unlikely to last long. Firms will either move up or down from this tier as the stratification of the market becomes more apparent. Berwin Leighton Paisner, after very strong growth over the past few years, is the most likely candidate for promotion to the next tier – if it can maintain that momentum.

The solid ones

As we work through the tiers, the differences between firms become less stark. The firms here are performing well, in some cases after a difficult few years.

This group will see considerable flux. Some may merge in the UK, others will contemplate a US merger (Richards Butler has already agreed to merge with Reed Smith) and some will drift off the pace. But the firms below this tier will face an even more uncertain future.

Unfortunately, in the legal sector there is no one killer statistic to use for ranking law firms. Above, we used PEP as a proxy for a well-run, profitable and well-regarded practice.

It is not quite that simple. The use of non-equity partners has a major distorting effect on profit per partner. In the magic circle, Slaughters is the most profitable firm and uses very few non-equity partners. Indeed, the tightness of its equity and the quality of its partners is its key selling point. If Freshfields Bruckhaus Deringer used non-equity partners to the same extent as Clifford Chance and Linklaters or perhaps applied harder performance criteria to its existing equity partners, based on its revenue per lawyer it would be significantly more profitable.

The recent announcement of the 'retirement' of 30 Freshfields partners should significantly address this issue.

Elsewhere the use of non-equity partners is more mixed; from CMS Cameron McKenna with none, to DLA Piper with plenty. To be a senior equity partner in DLA Piper it helps to be called Nigel.

Revenue per fee earner is a useful indication of the quality of a firm's practice, the price it can charge and the consistency of its pipeline of work.

We identified five tiers of firms on the basis of revenue per fee earner. From this table a number of factors are clear. In the magic circle, the sheer quality of the Slaughters offering shines through. As a relatively small firm it is not as tempted to do the more routine and price-sensitive work undertaken by its magic circle colleagues. The Linklaters efficiency drive of recent years has clearly borne results, but the rest of the magic circle are closely grouped. Macfarlanes is the only other firm that achieves comparable revenue per fee earner. For many firms the challenge is to significantly improve their revenue per fee earner. For firms seriously aspiring to be members of 'the stars' group, revenue per fee earner of £275,000 is necessary. It is noticeable how the use of non-equity partners, which can have such a profound effect on profits per equity partner, tends to unwind when looking at revenue per fee earner.

The most noticeable example is DLA Piper, which has strong equity partner profits but far less impressive revenue per fee earner of £203,500, significantly below the leading national firm, Addle-shaw Goddard, and below the recently troubled Hammonds.

These rankings are not just a means for partners too keep score with their colleagues in rival firms. They are becoming more relevant in the search for the best clients and for attracting and retaining the best legal talent. As client panel reviews become more rigorous, clients will increasingly look for firms that have the expertise they require, are stable and offer a pricing model appropriate to the work being outsourced.

For more complex work, the magic circle and star firms are likely to be approached first alongside those US firms with the necessary breadth and depth in London. With the better performing firms entering a virtuous circle of better work and better profitability, talented partners and associates will migrate to them.

Law firm leadership will need to be increasingly realistic as to their position in this fragmenting market. If they wish to move to a higher tier they will need to outperform the market consistently over several years. This will not be easy to achieve and will require a clear strategy executed with determination and rigour. It will be far easier to sleepwalk into relegation than to gain promotion to the next tier and to stay there.

Tony Williams is the founder of Jomati Consultants.