Soldiers posing as individual intruders or a gang of terrorists could be sent to carry out surprise break-ins at the castle and other royal palaces. They would be expected to look for weak points in security and try to get as far as possible into the castle or palace.

The SAS has previously been used by the Home Office to test the defences of highsecurity prisons.

Many of the police guards at royal homes are armed but the SAS men are not likely to be in danger. If police detect an intruder, they would challenge him and the soldier would reveal who he was. Police can open fire only if they believe that they, or others, are in immediate danger.

Outlining measures to tighten security after the fringe comedian Aaron Barschak broke into Prince William’s 21st birthday party on Saturday, Sir John said that he was also appointing a security chief to concentrate solely on royal residences. He admitted that much more should have been done to check the efficiency of the arrangements.

“There will be rigorous testing of our existing and future police arrangements,” he said. The tests would check “operational effectiveness including assessments on the ground. People testing what we are doing. People trying to get in . . . if necessary using the SAS.

“We are going to have regular testing arrangements to ensure we have a more effective and yet balanced security.”

At present, there are only a couple of exercises a year to test security at the castle.

Sir John said that an interim report by Commander Frank Armstrong would be on his desk by the end of next week. A full report will follow later but the new tests are already being planned.

The new security supervisor, who will be a chief superintendent or commander, will be appointed by Monday. They will report directly to David Veness, the assistant commissioner in charge of counter-terrorism.

Sir John said that the new co-ordinator would not take precedence over Commander Peter Loughborough, the overall head of royalty protection, but would work with him.

Scotland Yard is also creating a new intelligence database on individuals obsessed with the Royal Family who could cause security breaches. At the moment police have to rely on a list of 30 names and photographs that are routinely circulated among protection officers.

Now a fuller and much wider file is being built up which will also include publicity-seekers who might try to get close to the Royal Family.

Sir John said: “We want to focus more awareness on individuals who may be fixated, stalkers or publicity-seekers.” He said that no decisions on the future of Commander Loughborough, who is the 7th Earl of Rosslyn, and Chief Superintendent Peter Prentice, the head of royalty protection, had yet been made.

Sir John said that neither the commander nor the chief superintendent had offered their resignations and he had not asked for them.

• Eight officers based at Windsor Castle will today be told that they are under investigation for disciplinary offences over the incident at the party. They include the chief inspector in charge of security and PC Nigel Haspineall, who let Mr Barschak into the castle.

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