St Paul's Cathedral has been forced to close its doors to the public because of the anti-capitalist protest taking place outside.
The Dean of St Paul's, the Rev Graeme Knowles, said the decision to close the cathedral was made with "heavy hearts".
He said the decision was taken after church officials received a report by health and safety officials.
The Dean asked the activists, who have occupied a makeshift campsite outside the main entrance of the house of worship since Saturday, to move on.
He said the church would close its doors to members of the public after a service this afternoon.
The cathedral has not been closed since the Second World War, the Dean said.
"We have a legal obligation to keep visitors safe and healthy," he added.
Earlier in the week, a church official said the Occupy London Stock Exchange protest was causing a "risk to the life of the cathedral".
Mr Knowles, who has been forced to cancel a visit to America, said the decision was "unprecedented".
He said: "I have written an open letter to the protesters this afternoon advising them that we have no lawful alternative but to close St Paul's Cathedral until further notice.
"Health, safety and fire officers have pointed out that access to the cathedral is seriously limited.
"With so many stoves and fires and lots of different types of fuel around, there is a very clear fire hazard.
"Then there is the public health aspect which indeed speaks for itself. The dangers relate not just to cathedral staff and to visitors but are a potential hazard to those encamped themselves.
"The decision to close St Paul's Cathedral is unprecedented in modern times.
"We have done this with a very heavy heart, but it is simply not possible to fulfil our day to day obligations to worshippers, visitors and pilgrims in current circumstances."
Mr Knowles said he has asked the Registrar to implement "emergency procedures" to keep the building closed but fit for purpose.
The 200 staff and 100 volunteers were informed of the closure this afternoon.
A cathedral spokeswoman said staff would be coming to work "as usual".
The Dean continued: "In the open letter I am asking the protesters to recognise the huge issues facing us at this time and asking them to leave the vicinity of the building so that the cathedral can reopen as soon as possible."
He added: "I hope that the protesters will understand the issues we are facing, recognise that their voice has been legitimately heard, and withdraw peacefully."
He said the cathedral's Chapter defended the right to protest but would like to use the building for its intended use.
"That protest has happened, it has been legitimately heard and we would now like to be able to have space back and use it as we should," he added.
Some of the issues raised by Health and Safety and fire officers included the presence of unknown quantities of flammable liquids, smoking and drinking in tented areas, compromised fire exits and public health issues such as sanitation and food hygiene.
The cathedral is one of London's best loved tourist attractions and draws between 2,000 and 3,000 worshippers each Sunday.
He said small gatherings of no more than 100 people would still be allowed inside the church so planned weddings could still go ahead.
A cathedral dedicated to St Paul has stood on the site since 604 AD. The current building - the fourth to occupy the site - was designed by the court architect, Sir Christopher Wren, and built between 1675 and 1710 after its predecessor was destroyed in the Great Fire of London.
It is a key site on London's skyline, a popular location with tourists and is the cathedral of the Diocese of London.
It has been the focus point for many a royal celebration, including Queen Victoria's Jubilee and the marriage of Lady Diana Spencer to the Prince of Wales on July 29 1981.
As well as attending the wedding of Charles and Diana, the Royal Family has gathered in force at St Paul's over the years for the 80th and 100th birthdays of the Queen Mother and the thanksgiving services for the Queen's Golden Jubilee and her 80th birthday.
It also staged the funerals of Lord Nelson, the Duke of Wellington and Sir Winston Churchill, the peace services marking the end of the First and Second World Wars and more recently the Service of Remembrance and Commemoration for the September 11 terrorist attacks.
A group of around 200 activists living in the camp congregated on the steps of the cathedral for a meeting to discuss the closure.
The campers decided that they would not leave the vicinity "for the moment".
They took turns to address the crowd. One female protester said: "We should not be blackmailed into moving.
"We agree that we should stay. It is too necessary for us to go straight away."
Others chanted: "We should not go."
Activist Julian Stevenson, from Barnstaple in Devon, said it was "not constructive" to get involved in a conflict with the church.
Another said: "When we started this occupation, no one invited us."
One camper was holding a banner which read: "Where would Jesus be if he were here today? Would he be camped out in the freezing weather speaking out against inequality? Or would he be in a religious building worrying about revenue from tourists?"
A man, who did not wish to be named, read out a mission statement to the crowd. He said: "The current system is unsustainable, undemocratic and unjust.
"We stand together with occupations all over the world.
"We refuse to pay for the banks' crisis."
The camp, which has been erected for six days, is occupied by dozens of tents. The activists are camped under a banner which reads "capitalism is crisis".
Three generators are providing power to the camp, there are two portable toilets and a kitchen, and even a library has been constructed.