Executives at Barclays have donated over $1m to Romney's presidential campaign and will hand over more money on Thursday night at an exclusive fundraising dinner in a secret Mayfair location, where tickets cost between $50,000 and $75,000.
Romney arrived in the UK on Wednesday for a series of meetings with David Cameron and other political figures before attending two fundraising events and the Olympics this weekend.
An early-day motion (EDM) signed by 11 MPs last week demanded the bank and its directors stop working to bolster Romney's election campaign war chest and concentrate on repairing confidence and trust in the banking system instead.
But in a letter to the signatories of the motion, Cyrus Ardalan, a vice-chairman of Barclays and head of the UK and European government relations, said the bank was not a supporter of the presidential hopeful.
'I ... would like to clarify that all political activity undertaken by Barclays' US employees, including personal fundraising for specific candidates, is done so in a personal capacity, and not on behalf of Barclays', he wrote.
'Barclays is politically non-partisan, makes no political donations nor seeks to influence the political activities of its employees'.
The EDM, whose primary sponsor was Grahame Morris MP, criticised the fact that the recently departed Barclays chief executive, Bob Diamond, and other existing senior Barclays executives have played a prominent role in fundraising efforts for the Romney campaign.
The dinner was due to be co-hosted by Diamond but he pulled out after resigning as chief executive in the wake of the Libor rate-fixing scandal. Diamond has been replaced by Patrick Durkin, the managing director of Barclays Capital, the bank's "casino banking" investment arm.
Diamond, along with Cameron, was a very public supporter of John McCain, the last Republican candidate for the presidency, when he ran against Barack Obama in 2008. Romney earned a $50,000 speaking fee from Barclays in 2011.
MPs including Jim Cunningham, Mark Durkan, Margaret Richie and John McDonnell raised concerns that at least 15 of Barclays Capital's most senior bankers based in the US have donated the maximum allowable individual donation per election to the Romney campaign.
They also criticised the fact that Durkin has already reportedly raised over $1m for the Romney campaign.
Other hosts of the event, which is being held at a secret location understood to be a five-star hotel in central London, include Dwight Poler, the managing director of the European arm of Bain Capital, the private equity house founded by Romney, and Eric Varvel, the chief executive of Credit Suisse's investment banking arm.
Among 47 named co-hosts are Raj Bhattacharyya, a managing director at Deutsche Bank, Karl Peterson, the European boss of private equity firm TPG Capital. Also present will be Woody Johnson, the owner of American football team the New York Jets and a great-grandson of the founder of Johnson & Johnson.
Guests who can't quite stretch to the $50,000-$75,000 required for dinner with Romney are invited to a 5pm reception, which costs from $2,500 a head.
On Friday Romney will also attend the London 2012 opening ceremony.
The pro-Barack Obama super PAC Priorities USA Action will use his visit to the Olympics to release a new advert attacking Romney's record outsourcing jobs and moving millions of dollars of his wealth to offshore tax havens.
'We know the Swiss have a special place in Mitt Romney's wallet, er, heart', the voiceover of the advert says as the Swiss athletes march into the games with Romney shown waving. 'He kept millions in Swiss banks. Those Swiss sure know how to keep a secret'.
The advert ends with the line: 'You gotta say this about Mitt Romney. He sure knows how to go for the gold – for himself'.
While in London, Romney will also meet Cameron, Nick Clegg, George Osborne and Ed Miliband. It is the second time Romney has visited London to raise cash following a meeting with 'a few friends' in Mayfair last summer.
London's role in global finance has made it a key fundraising target in presidential elections. Obama, who is trailing Romney in the race for cash, held a similar fundraising party in the capital before the 2008 election.
Romney's event is being organised by Scott Preen, a London company that specialises in high-profile political fundraising parties. The company, run by Frances Penn, has previously organised lavish events for Romney and McCain, who ran against Obama in the 2008 election.
Scott Preen, whose clients include Bulgari, Dior and the National Bank of Greece, has previously held receptions at Spencer House, the 18th-century mansion owned by Lord Rothschild. The company declined to comment.
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