“Mr. Diamond decided to step aside as a co-host for the upcoming London reception to focus all his attention on Barclays . We respect his decision,” said Andrea Saul, a spokeswoman for Mr. Romney’s campaign.

Mr. Diamond had been one of 18 co-hosts for a dinner in London later this month in which guests are being asked to pay between $25,000- $75,000 to raise money for Mr. Romney, who will be in town for the opening ceremony of the Olympic Games.

Mr. Diamond is one of several top executives at Barclays who have thrown their weight behind Mr. Romneyto help defeat President Barack Obama. He also hosted similar events in support of John McCain during the 2008 presidential campaign.

At least 15 of Barclays Capital’s most senior bankers based in the US have donated $2,500 to the Romney campaign, the maximum allowable individual donation per election, Federal Election Committee filings show.

The executives include Tom Kalaris, chief of wealth and investment management, Marty Malloy, head of prime brokerage, Hugh “Skip” McGee, head of investment banking, Dean Maki, chief US economist and Gregg Chow, global head of FX.

Patrick Durkin, the British bank’s head of government policy and finance group, has raised $927,000 for Mr. Romney as one of the candidate’s top bundlers, supporters who collect donations from others. Mr. Durkin has also chaired six policy roundtables for Mr. Romney.

The support from Barclays executives is in line with the weight of Wall St money, which is backing Mr. Romney, unlike 2008, when Mr. Obama received the bulk of donations from the finance sector.

Mr. Romney’s visit comes just after Barclays was fined $450 million by the Commodity Futures Trading Commission for trying to manipulate a key bank interest rate which influences the cost of loans and mortgages.

The fine is part of an international investigation by UK and US authorities into the setting of interbank rates between 2005-2009.

Many Barclays executives have supported Republicans for office in previous elections also, though Mr. Chow donated to Mr. Obama in May 2007 during the last presidential election cycle.

Many bankers see Mr. Romney, the founder and former chief executive at private equity firm Bain Capital, as more sympathetic to their industry, especially on regulation and taxes. But many financial institutions still feature prominent Democratic executives that are backing Mr. Obama, including Robert Wolf, chairman of UBS Americas.

Mr. Romney has promised to try to repeal the Dodd-Frank law, passed by the Democratic-controlled Congress with Mr. Obama’s support in response to the financial crisis.

The banking industry has continued to press to minimize the scope of the law in the wake of its passage, while Republicans in Congress are trying to limit funding for its agencies. Barclays alone had spent $360,000 lobbying Congress as of April 12 this year.

Mr. Romney earned a $50,000 speaking fee from Barclays in 2011.