The firm, which also topped the table for the whole of last year, was boosted by advising Wellcome when it was acquired by Glaxo for pounds 9.15bn in Britain's largest public deal. Slaughter also defended Northern Electric against Trafalgar House's pounds 1.15bn attempted takeover.
Although the rankings have been distorted by the Glaxo-Wellcome deal, Philip Healey, editor of Acquisitions Monthly, points out that this year's trend is towards a substantial increase in the number of UK deals and their value.
For example, Freshfields' 11 deals equalled its total for the whole of 1994, while Allen & Overy's five deals put it ahead of its 1994 total of three. Moreover, the pounds 1.39bn total value of eighth-placed Allen & Overy's deals would have put it in second place at the halfway stage last year.
"Such activity augurs well for the mergers and acquisitions sector for the rest of the year and hopefully beyond, even though this is not likely to mean that the heady days of the late 1980s are about to return," Mr Healey said.
Indeed, the top two firms have got the second half off to a strong start by appearing in opposite sides of the pounds 977m takeover of Kleinwort Benson by Germany's Dresdner Bank, while Freshfields is advising Scottish Power in its pounds 1bn bid for Manweb, whose lawyers are Clifford Chance.
The Glaxo-Wellcome deal was the key to the top five positions in the table of advisers acting both for financial advisers and the companies themselves during the six months to the end of June.
Freshfields and Herbert Smith, which retained their second and third places from the full 1994 table, worked for the financial advisers of Glaxo and Wellcome, while Clifford Chance, which moved up from fifth to fourth, acted for Glaxo. Lovell White Durrant, which climbed from ninth in 1994 to fifth, represented Wellcome Trust, which held nearly 40 per cent of Wellcome.
One surprise entry to the list is Walker Morris, a Leeds-based firm which continues the recent trend for regional firms to pick up the occasional corporate appointment at the expense of City practices. Past entrants include Hammond Suddards and Dibb Lupton Broomhead, which are also based in Leeds.
Walker Morris's ninth position after not ranking last year is largely on the back of advising the Dewsbury-based paints group Kalon on the acquisition of Euridep from the French oil group Total. The pounds 530m deal - part of a series of transactions worth nearly pounds 570m - is said to be the largest to be handled by a law firm outside the City.
On the other hand, many observers will be surprised at the continued poor showing of Linklaters & Paines, which is generally reckoned to be one of the leading three firms on account of its top-flight client list. With just three deals worth a total of pounds 1.9bn, it retained the sixth position it held at the end of last year.
Meanwhile, there is also a continued improvement in the management buyout and buy-in market. Preliminary figures for the first half of the year just published by the Centre for Management Buyout Research show the value of deals higher than at any time since 1989 at pounds 2.4bn.