A US campaign watchdog has accused presumptive Republican president nominee John McCain of violating election laws by accepting campaign contributions from two prominent Londoners.

At issue is a fundraising luncheon held in March at London's Spencer House, during McCain's swing through the United Kingdom. An invitation to the event lists Lord Rothschild and Nathaniel Rothschild as hosts, and indicates the event was made possible with their "kind permission".

Judicial Watch, a Washington organisation instrumental in the March release of Hillary Clinton's White House schedules, has asked US election monitors to investigate whether the Rothschilds improperly sponsored the fundraiser. US political campaigns are forbidden from accepting contributions from foreign nationals.

"The question is whether or not the Rothschilds paid for the event, the venue, the catering, or any other related costs," said Judicial Watch president Tom Fitton.

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