Orrick Herrington & Sutcliffe and Pillsbury Winthrop Shaw Pittman have called off merger talks, citing irresolvable client conflicts.

If merger talks had been successful Orrick Pillsbury would have become the ninth largest law firm in the US, with a combined revenue of $1.4bn (£866m).

The decision to call off negotiations was reached before either firm had signed a letter of intent or put the proposed tie-up before their respective partnerships for a vote.

Both firms confirmed they were in talks last month, though the discussions are understood to have begun in the summer.

In a statement, the two firms said: "We mutually determined that we will not be able to proceed due to prospective client conflicts that we have not been able to resolve, notwithstanding each firm's best efforts.

"Our two firms come away from these discussions with enormous respect for each other both institutionally and across our management teams, a mutual respect that has only grown throughout our discussions."

An Orrick spokeswoman confirmed to Legal Week that the conflict involved Pillsbury's tax controversy, environmental and real estate practices and Orrick's public finance practice.

"Pillsbury's corporate clients' interests are sometimes adverse to the interests of the state and municipal entities that Orrick represents," she said.

Despite the failure, the respective chairmen issued glowing appraisals of the other firm's lawyers.

"Pillsbury has long been known to us as a firm with high calibre legal talent and a client-focused culture much like ours," commented Orrick chairman Mitch Zuklie. "Large law firm combinations are always complex, and both our firms are disappointed that we could not clear the way for a merger."

Pillsbury's Jim Rishwain added: "Orrick is a firm with a longstanding and well-deserved reputation for legal excellence. Throughout these discussions, our admiration for Orrick's attorneys has only deepened."

Orrick posted a revenue of $866m (£534m) in 2012, while its 124 equity partners earned an average of $1.63m (£1m).

Pillsbury is the smaller of the two firms, having generated a revenue of $561m (£346m) and profit per equity partner of $1.1m (£679,000) last year. The firm has 609 lawyers, which includes 163 equity partners.

Orrick's key clients include Apple and Microsoft, while Pillsbury's client base includes BNY Mellon, Chevron Corporation and Amazon.

The failure of the talks is the latest in a string of unsuccessful courtships involving Orrick, and follows merger discussions with Bird & Bird and SJ Berwin in the UK as well as legacy Dewey Ballantine in 2006.