The leader of the local police authority said yesterday that that the marriage could put financial pressure on already limited resources.

Clarence House said that a decision on whether the Prince of Wales and Camilla Parker Bowles would walk or go in a motor cavalcade the 100 yards from Windsor Castle to the Guildhall and back would depend on security advice.

The Queen is deeply concerned at the shambolic preparations for her son’s second wedding. Clarence House had to abandon its original plans, announced only a week before, after discovering that the licence permitting a civil ceremony at Windsor Castle would run for three years. It would have enabled commoners to enjoy the same right as the Prince of Wales to marry in the castle, which is the Queen’s favourite residence.

An inquest is under way at Clarence House to pinpoint how the usually meticuluous planning for such a formal public occasion went awry. Clarence House decided not to seek advice in advance from Windsor council for fear that the wedding details would leak.

The Prince of Wales, who is embarrassed by the debacle, has made clear his displeasure. “Clearly this has not been the happiest here,” an official at Clarence House said.

The first that Thames Valley Police, which is responsible for Windsor, knew about the move was when it was announced on Thursday. A wedding behind the battlements of Windsor Castle, with some 700 guests, including the Prime Minister and other political leaders, would have created minimal security concerns. The Metropolitan Police are responsible for security inside the castle.

But now Thames Valley police will have to make sure that the route between the castle and the Guildhall is safe and swept for possible explosive devices. Police snipers will be positioned on rooftops and helicopters will hover. The costs will be borne by the local council taxpayers. The new venue is surrounded by restaurants and pubs, one of which was used by Aaron Barschak, the “comedy terrorist”, to launch his assault on the castle when he gatecrashed Prince William’s 21st birthday party.

A royal source said: “Inside the castle we knew exactly who was where. Now the wedding has been moved into the town centre we have lost a lot of control. This is not what was itended by a low-key, tightly controlled wedding.”

The Prince of Wales was planning to invite 700 guests to his second wedding but the Guildhall has a capacity of only 200. The guest list will either have to be cut back or a two-tier system will be introduced, with the most important going to the civil ceremony and the rest being invited only to the blessing in St George’s Chapel.

The latest public order costings from Scotland Yard show that policing the Festival of Remembrance at the Albert Hall, for instance, costs more than £200,000 and involves 700 officers. The Remembrance Day observances the next day costs Scotland Yard more than £430,000 with the deployment of 1,400 officers.

Sally Hannon, the chairwoman of the Thames Valley Police Authority, said that the cost of the operation would be met out of contingency funds. She said that some members of the authority were worried that they would face financial difficulties. Mrs Hannon said it was bizarre that courtiers had not realised earlier that having the civil ceremony in the castle would mean that members of the public could also use it.

The document announcing their intention to marry at Windsor on April 8 was posted on the notice board at Cirencester Register Office, about ten miles from the Prince’s Highgrove House estate in Gloucestershire.

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