Norton Rose is teaming up with US firm Skadden Arps Slate Meagher & Flom to complete Citigroup's billion pound takeover of Schroders' investment arm.
The UK firm won a place advising on the £1.35bn acquisition through a recommendation from US counsel Skadden Arps.
Usually, Citigroup uses Freshfields and Clifford Chance for major deals in the UK but Norton Rose worked with Skadden Arps on US-based Cendent's abortive attempt to take over the RAC last year.
The acquisition was made by Citigroup's investment banking subsidiary Soloman Smith Barney and agreed last week.
Barbara Stephenson, corporate finance partner at Norton Rose, who headed the team of 32, including eight partners, says the firm was officially chosen to work for Citigroup only a week before the sale was agreed.
She says: "We were cleared for conflict on Monday, by Tuesday we were told we were acting on the deal."
Stephenson says she believes no other firms were conflicted out.
Although the sale has been agreed, she says there are still several regulatory issues outstanding. "It is going to take ages to get regulatory consent in each of the countries where Schroders has a presence," she says.
Michael Hatchard, corporate partner at Skadden Arps, is the only UK lawyer working on the deal for the firm.
Skadden Arps' team of five includes US-based tax partner Stu Finklestein and corporate partner Eric Friedman.
On why the US firm referred a chunk of the UK work to Norton Rose, Hatchard says: "We are a relatively small presence [in London]. Our role in a transaction like this is not to be the authoritative domestic firm.
"We deal with these transactions in both a US and UK capacity. But if there is specialist domestic tax advice required we will approach the right local firm."
Glen James, corporate finance partner at Slaughter and May, headed a team including corporate partners Martin Hattrell and Oliver Wareham acting for its long-time client Schroders. The firm is being assisted on US law by Sullivan & Cromwell.
Of the deal's complexity, Hattrell says: "I wouldn't say it was run of the mill. It happened quickly and it had a complex structure.
"It is unusual with a plc that the proceeds of the sale of the disposal were given directly to shareholders. They usually go to the plc."