Slaughter and May is the leading adviser to the FTSE 100, acting for 33 clients in Q3 of 2015. It also tops the table for FTSE 250 clients having advised 44 over the three month period.

Linklaters is second in terms of FTSE 100 clients, with 30, followed by Freshfields Bruckhaus Deringer with 24 and Allen & Overy with 21.

Slaughter and May's head of corporate Andy Ryde said: "We take a long term view about rankings but, of course, it is reassuring that we continue to top the tables for numbers of clients in the FTSE 100, the FTSE 250 and across the listed market. If our lead in those numbers looks comfortable, believe me it doesn't feel comfortable and is the result of some very hard work."

The rankings, compiled by finance data provider Adviser Rankings using Morningstar data, show that Linklaters remains in top spot when market capital of FTSE 100 clients is taken into account. It acts for FTSE 100 clients with a combined market capitalisation of £756.204bn, beating Freshfields into second place with £573.760bn, followed by Slaughter and May on £543.971bn and Allen & Overy on £484.405bn.

Linklaters London senior corporate partner Charlie Jacobs said: "In the 25 years I have been at Linklaters, Linklaters and Slaughter and May have always had the most FTSE 100 clients and typically at the higher end of the market capitalisation, despite a lot of new entrant law firms. The fact we are both still there is a testament to the strength and depth of both of our corporate practices."

In July Linklaters acted for FTSE 250 chemical company Alent on its $2.1bn (£1.3bn) takeover by US company Platform Specialty Products.

Meanwhile recent deals for Slaughters include the firm's role advising FTSE 250 company Ladbrokes on its merger talks with fellow gambling company Gala Coral.

The data, which focuses on the three months to 5th August 2015, also shows that Slaughters topped to the rankings for total number of stock market clients with 117, a drop of two since the last quarter. Pinsent Masons (which topped the AIM rankings) is second with 100 and Herbert Smith Freehills moves into third having added four clients over the period to reach a total of 92 clients. Herbert Smith also added one FTSE 100 client in the quarter, taking its total number to 18.

Pinsent Masons remains top of the AIM rankings table but its position could be threatened from September when Gowlings London office joins up with Wragge Lawrence Graham & Co as a result of the merger of those two firms. Pinsents currently has 59 AIM clients, Wragges has 39 and Gowlings 21.

Wragges' chief executive David Fennell said: "The Gowlings London team has a fantastic mining and natural resources client base. If you add that to the AIM practice we have got that will definitely make us the number one law firm on the AIM market."