New York firms Brown & Wood and White & Case are in merger talks to create a 1,214-lawyer firm - the fourth largest in the US.
If the deal goes ahead, White & Case has said the merged firm will look to the UK for a merger partner.
John Bellhouse, White & Case executive partner in London, confirmed that negotiations were taking place, but emphasised that they were still at a "preliminary" stage.
He said: "The rationale of the talks is the need to enhance our capital markets practice in the States and so enhance our attractiveness as a merger partner in the UK."
White & Case is thought to have been in negotiations with Norton Rose and Allen & Overy earlier this year, but both sets of talks appear to have stalled.
Thomas Smith, chairman of Brown & Wood, categorically denied reports that his firm had approached Clifford Chance in New York before talking to White & Case. "We have not approached any English firms," he said.
Tony Williams, Clifford Chance's managing partner, said: "It has been known that Brown & Wood has been looking at its positioning for some time and has been looking at the UK market, but it has not approached us."
One senior partner at another top five City firm said: "I'm not sure [the merger] would change things, as neither firm is seen as a top 10 firm in US securities."
Bellhouse said that Brown & Wood's Beijing office was also of interest to his firm, as White & Case has yet to be granted a licence to open an office in the country.