Nearly £3 million was spent on overtime costs alone, with hundreds of officers drafted in to help police crowds watching the event in London in 2011.

While next month’s wedding of Prince Harry and Meghan Markle at Windsor Castle  will be on a smaller scale, Thames Valley Police are preparing for around 100,000 spectators.

That will make it one of the force’s largest security operations, which will require reinforcements from other forces, including the Metropolitan Police.

Prince Harry and Meghan Markle take their seats at a star-studded concert to celebrate the Queen's 92nd birthday at the Royal Albert Hall. Picture: John Stillwell

Ken Marsh is chairman of the Metropolitan Police Federation, which represents rank and file officers in London.

He said the extra hours required of officers for a combination of large operations and an “unprecedented” level of violent crime in London have become a “huge drain” and must be seen as part of a wider welfare issue which is putting front-line services at risk.

Figures obtained through several Freedom of Information requests showed that, in total, £6.35 million was spent policing Kate and William’s wedding, including £2.8 million on police overtime.

Of that, £3.6 million was paid by a Home Office grant to cover “additional costs”, the Metropolitan Police said.

An estimate for the total cost of the security operation put forward by the Metropolitan Police in 2011 placed the figure closer to £7.2 million, according to a previous Freedom of Information disclosure.

And while the UK’s national tourism agency Visit Britain said the country experienced a 7 per cent increase in visitors between April and June 2011 compared to the previous year, those hoping for a boost to the wider economy may have been left disappointed.

Figures from the Office for National Statistics (ONS) show the combination of an extra bank holiday and the warmest April for 100 years contributed to a slight dip in economic activity.

This included a 1.2 per cent fall in output in service industries, a 1.4 per cent drop in manufacturing production and a 1.6 per cent fall in the index of production compared with the previous month.

The policing cost figures come ahead of preparations for the next royal wedding on 19 May, and a row over police staffing levels amid claims that the spike in killings in London is linked to cuts in police numbers.

Mr Marsh said: “Obviously, this is a huge drain on my colleagues in terms of the hours that they have to work.”

Security for royal wedding cost more than Â£6 million