Security chiefs and Buckingham Palace have withdrawn Royal Guards behind railings and fences, amid fears of a 'lone wolf' terror attack on one of the ceremonial troops responsible for manning positions at the Royal Palaces.

The guards are trained soldiers, but their weapons are usually unloaded and they are believed to be potentially vulnerable to a terror attack. The move comes in the wake of calls by Islamic State representatives for strikes against Britain by followers, while groups linked to Al Qaeda have encouraged supporters to take part in individual actions against the UK.

It is believed that intelligence agencies may have picked up internet 'chatter' about the highly visible Royal Guards, with their bearskin hats and scarlet uniforms, and they are thought to be at risk as terrorists may see them as relatively unprotected, yet high profile targets.

Guards have been moved from the front gates of St James's palace in London to a courtyard within the palace, while others at Prince Charles' official residence Clarence House have been relocated along with their sentry boxes from a position outside railings to a new site within the gates.

Meanwhile armed police unit have now been installed to guard the guards at other locations, including Windsor Castle, where Life Guards on foot and horseback - a major draw for tourists - are now protected by up to four Metropolitan Police officers armed with assault rifles and Glock pistols.

At Windsor, a lone sentry previously manned an exit used by tourists in a sentry box. Now this position is also guarded by at least three armed police officers.

In most cases the changes have significantly reduced the visibility of the troops to tourists and other visitors.

The move is thought to be in response to the killing of soldier Lee Rigby outside Woolwich Barracks in May last year as well as the shooting of Canadian sentry Nathan Cirillo, 24, outside the Ottawa National War Memorial, by mentally disturbed drug user Michael Zehaf-Bibeau, who had recently converted to Islam.

Chloe Howard, deputy editor of royal family blog Royal Central, said: "It is a shame as it denies the public the chance of seeing these well-known uniforms that are part of the British identity. Some may think it is a retreat."

A spokesman for the Ministry of Defence said: "We routinely review security arrangements."