Britain's Prince William (L) and his girlfriend Kate Middleton are seen attending the Six Nations international rugby union match against Italy in London in this February 10, 2007 file photograph.[Photo/Agencies]

Britain's Prince William walks with his girlfriend Kate Middleton at RAF Cranwell in this April 11, 2008 file photo.[Photo/Agencies]

Palace officials said only that the wedding would be held in London. It was too early to estimate its cost.

Mark Niemierko, a luxury wedding planner based in London, said a high-end ceremony in the capital typical costs from a quarter-million pounds to a half-million pounds. ($397,000 to $794,000)

Funds will likely come out of the Civil List — money provided by Parliament to meet official royal expenses, the queen's household allowance, or drawn from her personal wealth. That is, unless Parliament votes to give the royal couple extra money for the wedding.

Taxpayers will, at the least, have to pay for security at the event, which will require large numbers of police officers.

Jenny Jones, a Green Party lawmaker at London's City Hall, said the royal family should be asked to contribute to the likely high cost of policing the wedding.

"In this age of austerity, it's unrealistic to expect the taxpayer to pay millions of pounds for policing a wedding, however beautiful. We can keep costs down by making it a low-key event or the royal family can contribute. That would seem the fairest solution," she said.

Others refused to let financial concerns mar a nearly perfect day.

"We've been waiting a long, long time for this," said Felicity Thompson, who rushed to St. James' Palace clutching a newspaper with a front-page report about the engagement. "It's a piece of history."