Clifford Chance managing partner Geoffrey Howe unveiled the firm's strategy for the 21st century at an American Bar Association seminar on global practice.
He said City firms would have to look abroad increasingly because of the unreliable long-term condition of the UK economy.
"There will be fewer firms offering a lower cost to the client," he said. "The importance of international securities markets makes it vital for us to develop specialist knowledge of US law.
"Internationally, we have to develop the Clifford Chance brand name and its reputation for local expertise in each country. Simply to have a base in a country is not enough. We do not have representative or branch offices. We want each office to operate as a competitor in its own right.
"There is no point taking the time and trouble building international offices if we just offer what is already available from local firms already."
Howe demanded "total commitment" from partners. "Lawyers are naturally conservative, but the best solicitors and clients will only come to those firms which welcome change and do not hanker after the 'good old days'," he said.
He said the firm has no plans to break into the US and challenge domestic firms.
Speakers from both sides of the Atlantic said the US is extremely competitive. Both US and UK firms are looking to the developing markets of the world to escape the problems of their own markets.
Ronald Beard of Los Angeles firm Beard, Gibson, Dunn & Crutcher, said: "The recession masked the real issues affecting law firms, which were structural. What we need now are new markets and strategic planning."
He said the size of the American legal market had risen fourfold to $100 billion since the early 80s, while the number of lawyers has increased threefold.