Allen & Overy and Slaughter and May are pushing through the merger of United News & Media and Carlton Communications - which could result in the slashing of law firms used by the companies.
The proposed merger will form a £7bn group which plans to review its arrangements with outside law firms on completion.
A source close to the talks tells The Lawyer that there are a number of integration issues yet to be resolved, including the future outsourcing of legal services.
He admits this could lead to the slimming down of the number of advisers currently working with the two separate companies.
United News & Media has a panel of four firms - A&O, Ashurst Morris Crisp, Berwin Leighton and Macfarlanes.
Carlton does not have a fixed panel, but, in addition to Slaughter and May, has in the past used Clifford Chance, Freshfields and Bird & Bird.
Carlton company secretary David Abdoo says: "We don't have a panel as such - we use horses for courses."
When asked about a future panel review he will not comment, but confirms it will be an issue.
The Slaughter and May team working on the merger is headed by corporate partner Stephen Cooke. He is supported by competition partners Philippe Chappatte and Laura Carstensen, tax partner Tony Beare, and eight assistants.
At A&O, corporate finance partner Charles McKenna is leading the team, assisted by corporate finance partner Mark Gearing and 12 associates. Ashursts company partners Tony Ghee and Roger Finbow are advising United News on regulatory and competition issues.
Abdoo says: "This has been a very successful deal.
"We have all worked fairly quickly and it has been quite high pressure, but we have had an orderly run up to the announcement.
"We now have to follow that up with all the formal documentation and the regulatory aspects. It should all finally go through in the next couple of months."
If the deal is agreed by the Independent Television Commission and the Office of Fair Trading, it will create one of the largest media groups in the UK, combining a number of regional television stations and national tabloid newspapers under one umbrella.