The United States had wanted to extradite Stuart Scott, HSBC's former head of cash trading for Europe, the Middle East and Africa, to face fraud charges over allegations he and a colleague defrauded client Cairn Energy Plc in a $3.5 billion (£2.7 billion) currency trade in 2011.

Scott took his case to London's High Court in the hope of overturning an earlier court decision to send him to the United States in a case that has already seen his colleague, Mark Johnson, sentenced to two years in prison.

The judge, Lord Burnett, ruled in favour of Scott's appeal, saying it was not in the interests of justice for him to be extradited because most of the alleged crimes took place in Britain and because Scott has no significant connection to the United States.

"Conversely, those factors which told in favour of extradition were of significantly less weight," he said in a written ruling.

"In these circumstances this court is entitled to say that the judge's overall evaluation was wrong ... the appellant's extradition is not in the interests of justice and the appeal should be allowed."

The court's rare decision to block a U.S. extradition request for now allows Scott to avoid the fate of Johnson, who became the first banker to be tried in the United States as a result of worldwide investigations into the multi-trillion-dollar per day currency market. He was convicted in October.

The U.S. Justice Department declined to comment.

U.S. prosecutors allege that Scott and Johnson, formerly head of HSBC's global foreign exchange cash trading desk, schemed to ramp up the price of British pounds before executing the trade for Cairn, making millions for HSBC at Cairn's expense.

Cairn had hired HSBC in 2011 to convert $3.5 billion into British pounds sterling in connection with the sale of an Indian subsidiary.

Scott's lawyer, Anne Davies, said he was "very pleased" with the court's judgment, but added that the United States has indicated an intention to appeal.

"As proceedings are ongoing it would be inappropriate to comment any further," she said in a statement.

Scott's lawyers argued that the extradition request misrepresented the relationship between HSBC and Cairn, which they said had made clear HSBC was not acting as an agent of the firm and did not owe it any fiduciary duty, and that no crime had been committed under English law.

HSBC declined to comment. The bank agreed to pay Cairn $8.08 million under a settlement reached in July 2017.

(Reporting by Emma Rumney, additional reporting by Nate Raymond in Boston; editing by David Evans)