500 staff, armoured limousines, and a fleet of decoy helicopters... Barack Obama set to sweep into UK under huge security

A massive security operation is underway to protect Barack Obama on his first visit to Britain as president.

An unprecented entourage numbering more than 500 people is to accompany the American leader when he arrives in London on Tuesday, to ensure his safety, and his ability to operate as head of state 3,000 miles from the White House.

Such a group has rarely surrounded a foreign leader on a visit to these shores since the days of the Roman emperors.

Security is an acute concern for Obama's advisers as he arrives for the G20 conference being hosted by Gordon Brown in London on Wednesday and Thursday.

Big razzmatazz: Barack Obama steps off Air Force One which is scheduled to land at Stansted on Tuesday

A repeat of the relaxed scenes for the G8 summit in Birmingham in 1998 when the then President Bill Clinton shared a bowl of chips with local pub-goers during an impromptu walkabout appears out of the question.

Assassination attempts are feared possible not only from Al Qaeda, and its Britain-based supporters, but also from extreme racists enraged by the election of black president.

 As a result, 200 of the U.S. state employees accompanying the president will be members of the secret service, prepared at all times to take a bullet to save Obama's life.

The president is due to fly in to Stansted airport in Essex  on Tuesday in his official jet, Air Force One - which boasts technology to protect its communication systems from nuclear fallout and  radiation, as well as a gym to keep the president fit.

White House cavalry: Security officers such as these two men will be ready to take a bullet for Obama

A helicopter, Marine One, will then fly Obama to London - possibly amid decoy helicopters to confuse assailants - where he is staying at the U.S. Ambassador's fortified residence at Regent's Park in North London.

When travelling around London, the new president will be encased inside Cadillac One, his heavily fortified official car, complete with bullet-proof windows, night vision cameras, and pump action shotguns in case of emergency.

And surrounding him at all times, as unobtrusively as possible, will be his mini-army of secret service agents, sporting Ray Ban sunglasses, and communicating with each other by shirt-cuff radios and earpieces.

If the worst were to happen - along with previous successful assassinations of presidents there was a failed attack on Ronald Reagan in Washington DC in 1981 which left a press secretary paralysed by a gunshot - a White House medical team is on hand, too.

Rather than being entrusted to the National Health Service, Obama would be cared for a specially picked unit of surgeons and nurses, carrying them with supplies of type AB blood.

A total of almost 300 officials from state departments will also be on hand, to ensure the president can, from London, cope with any sudden crisis.

But two individuals among the group are perhaps more significant than any others. One is an unnamed military officer carrying the codes necessary to launch a nuclear missile attack.

The other is Obama's personal aide Reggie Love, who often starts the day by exercising with the president and ends it watching sports matches with him, in between operating the president's BlackBerry and handing him chunks of nicotine gum.

Relaxed: When former U.S. president Bill Clinton was in Birmingham in 1998, he mixed with the locals at a pub - it's a very different story for the current president

He will also be handing the president the basketball he will require for his daily game on the ambassador's court.

First Lady Michelle Obama will also have her own team of eight staff, including a press officer, a secretary, and her own detachment of bodyguards.

One US government source said: 'When the president travels, the White House travels with him, right down to the car he drives, the water he drinks, the gasoline he uses, the food he eats.

'America is still the sole superpower and the president must have the ability to handle any crisis, anywhere, any time.'

To guarantee the president's trip runs smoothly and safely, security officers have already made three reconnaissance trips to Britain, with special sweeps for electronic bugs and lethal bacteria at key locations last week.

Assuming all has gone to plan, Obama is due to leave early on Thursday or late on Friday - accompanied by his massive team.