Israel visit ... Mitt Romney meets Benjamin Netanyahu. Israel visit ... Mitt Romney meets Benjamin Netanyahu. Photo: Getty Images

NEW YORK: It is on course to become the most expensive election in history, with at least $US3 billion ($2.86 billion) likely to have been spent by the end of this year's battle for the White House.

So even though Mitt Romney may have had an uncomfortable few days in the spotlight while visiting London, the good news for his campaign was that he was putting dollars in the bank.

The Republican challenger for the US presidency managed to annoy his hosts by questioning London's preparedness to host the Olympic Games but, in the race with the President, Barack Obama, for money, London was nonetheless a lucrative pit stop.

Mr Romney added a further $2 million to his coffers at a $75,000-a-head dinner for affluent US expatriates, held at the Mandarin Oriental Hotel on Thursday.

But the funds raised in Britain are nothing compared with contributions from donors such as Sheldon Adelson, one of the 10 richest Americans, who are willing to write $10 million cheques as down payments for the cause. The Las Vegas casino magnate was chosen to be in the welcoming party of wealthy Jewish-American supporters of Mr Romney during his visit to Jerusalem at the weekend.

Meetings with the Prime Minister of Israel, Benjamin Netanyahu, an old Romney managing consulting acquaintance from Boston, and Palestinian leaders were top of the official agenda for the former governor of Massachusetts.

But in a fund-raising event at the King David Hotel, Mr Adelson was due to be the star turn. The Zionist billionaire and his Israeli wife, Miriam, have pledged to donate $100 million to Republican groups to fight November's elections and gave $10 million last month alone.