Fears of a terrorist attack on soldiers guarding Buckingham Palace has prompted the security services to employ anti-bomb equipment during the nation's most famous royal ceremony.
At least two soldiers taking part in the Changing of the Guard now carry backpacks, hidden inside their uniforms, which contain a state-of-the-art device used to detect and block radio waves transmitted to detonate bombs.
The piece of equipment is thought to be the enhanced replacement for the original Electronic Counter Measures (ECM) pack, which was developed to protect soldiers from IRA bombs detonated using radio frequencies.
Security sources say the measures reflect the high state of alert surrounding London following the suicide bomb attacks in July.
"We have implemented covert security measures across London and in particular at what we see as high-profile terrorist targets," one source said.
"Buckingham Palace and the Changing of the Guard is just one such site."
Hundreds of thousands of tourists visit the palace every year to see the ceremony, which dates back to 1660.
The Changing of the Guard takes place in the forecourt of Buckingham Palace at 11.30am daily in summer, every other day in winter, and lasts 45 minutes. The new guard marches to the Palace from Wellington Barracks with a Guards band. The old guard hands over in a ceremony during which the sentries are changed and then returns to barracks. The new guard then marches to St James's Palace, leaving the detachment at Buckingham Palace.
In September 2001, two days after the terrorist attacks on New York, a special ceremony included, for the first time ever, the playing of the US national anthem.
Security sources also explained how they have sought the co-operation of mobile phone companies to shut down their networks during terrorist alerts.
This is to prevent would-be attackers detonating bombs using mobile phones similar to the attacks in Madrid last year, which left 191 people dead and 1,460 others injured.