BOSTON (Reuters) – Republican presidential hopeful Mitt Romney takes his fund-raising machine to London next month, holding a reception aimed at wealthy American expatriates working at banks and hedge funds.

A handful of London fund-raisers were held during the 2008 presidential election cycle, but Romney's event, scheduled for July 6, will be the earliest ever of its kind.

The reception will be held at Dartmouth House, an opulent mansion in the heart of Mayfair that is often used for weddings and conferences.

The suggested contribution to the "Romney for President" campaign is $2,500, according to an invitation seen by Reuters. Under U.S. law, only American citizens and green card holders can contribute.

Romney is the front-runner for the Republican nomination to take on President Barack Obama in 2012, in part because of his campaign's huge war-chest. He also ran unsuccessfully for the Republican presidential nomination in 2008.

The former Massachusetts governor has held dozens of fund-raisers across the United States this year, including at least three in New York that were hosted by backers from the financial industry.

Co-chairs of the London event include billionaire hedge fund manager Louis Bacon, founder of Moore Capital Management; businessman Robert Wood "Woody" Johnson IV; and Dwight Poler, managing director for Europe at Bain Capital, the venture capital firm that Romney co-founded in 1984.

Overseas fund-raisers were unknown for U.S. candidates until former New York Mayor Rudolph Giuliani, at that point among the top Republican candidates, broke the ice in September 2007.

John McCain, who eventually won the Republican nomination in 2008, also raised funds in London. And Obama raised a reported $400,000 at the London home of Elizabeth Murdoch, daughter of media tycoon Rupert Murdoch, in May 2008.

Romney campaign officials did not immediately comment on the candidate's plans while in London.

(Reporting by Ros Krasny; Editing by Paul Simao)