The arrest of a man armed with a knife in an Australian convention centre where the Queen was to give a speech has again raised the issue of royal security.

The Queen, like many members of the Royal Family, is provided with a 24-hour guard by officers from the elite Royal Protection Squad.

Both Scotland Yard and Buckingham Palace are reluctant to talk openly about the squad, also known as Special Operations 14 or SO14, for fear of compromising their close protection role.

The squad is a part of Metropolitan Police's Royalty and Diplomatic Protection Department, which employs 397 officers and 47 civilians.

Around 50 bodyguards are reportedly detailed to protect high-profile royals. These personnel are drawn from Metropolitan officers with "some years of operational street duty", according to Scotland Yard.

Members of the squad, both uniformed and plainclothes, receive specialist training in unarmed combat and marksmanship.

Bullet catchers

Uniformed officers perform the "less immediate" role of guarding royal palaces against intruders.

Other officers, the so-called "bullet catchers", are detailed to "protect the person" of the Queen.

Reputed to be armed with Glock 9mm automatic pistols, these officers are the last line of defence should an attacker or terrorist break through the outer rings of security.

As well as accompanying the Queen on official engagements, like her current tour of Australia, these close protection officers are even said to stand guard over the Queen's bedroom.

Several top royals, including the Queen, are reported to find "presidential style" heavy security a hindrance to their official duties.

Despite being heir to the throne, the Prince of Wales favours being accompanied by a mere four bodyguards.

In addition to the Royal Protection Squad, the Special Escort Service provides added security when royals are on the move.

The service employs highly skilled drivers and motorcycle riders, complete with a fleet of specially armoured royal cars, including a modified Rolls Royce.

Travelling light

The Queen, Prince Philip and Prince Charles often opt for a low-key approach to travel arrangements, doing without motorcycle outriders.

The Queen is also reportedly to be keen to reduce the �30m annual bill for royal security. A recent report by Her Majesty's Inspectorate of Constabulary even suggested creating a slimmed-down civilian service.

Although the Northern Ireland peace process has reduced fears of a terrorist attack on the Royal Family, there remain concerns about extremists and the mentally ill.

More than 6,000 mentally unstable people have persistently visited royal palaces or written to the Royal Family over the last six years, according to a police report.

Dr David Enoch, a senior consultant psychiatrist, say this attention could lead to violence.

Hate mail

"They might write to the Queen with grievances against the country, or even their neighbours. If their claim is rejected, or isn't answered, they project their anger onto the Queen."

There have been calls for protection squad officers to receive psychological training, to help them spot a mentally ill individual who might pose a threat to the Queen.

Clive Hudson from Intelligence Security Executive says forward planning is the only way to prevent an attacker reaching a member of the Royal Family.

"You can't accuse someone of a terrorist act just because they're standing in a crowd looking shifty."

Mr Hudson says the Royal Protection Squad officers with the Queen in Australia will have conducted a study of the convention centre where the man was arrested.

Forward planning

The Queen's bodyguards will familiarise themselves with the building's layout, escape routes and access to medical facilities.

Officers from the local police force will check sewers, drains and rooftops for bombs or potential assassins.

It seems the operation to guard the Queen offered an "onion layer" of protection, with light security around the building and a heavier presence closer to the Queen.

The outer layers of security would probably be made up of uniformed officers and undercover police in the crowd, watching for possible trouble.

So-called "rummage squads" of uniformed officers would also tour the building, checking and sealing off risk areas.

Once an area is secured, anyone entering that part of the building would come under immediate scrutiny and, as was the case with Gregory Pailthorpe in Sydney, be arrested.

"Once you cross that line, that's it," says Mr Hudson.