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.

Tickets to the event cost $1,000 to $2,300, and the luncheon dress code was "lounge suits," the Washington Post reported in March.

The McCain campaign did not immediately return a call seeking comment.

Judicial Watch also complained to the US election authority, the federal election commission, about Elton John's involvement in Clinton's campaign. The group alleged the British rocker broke US campaign laws by performing at a fundraiser for the New York senator. The Clinton campaign argued US law allowed the British musician to volunteer his time and solicit Americans for contributions.

The federal election commission is unlikely to act soon on the group's McCain complaint. It is currently short-handed, a result of a political squabble between the Democratic-led senate and the Bush administration, and lacks a quorum to take action.