Conservation experts from the United Nations are visiting the Tower of London to judge whether it should be placed on its Heritage in Danger list, Unesco has said.
Unesco, the UN's cultural organisation, fears that the 900-year-old Tower has become so overshadowed by skyscrapers and other modern buildings that its historic value is being damaged.
They have warned the UK to make greater efforts to protect the Tower, which is one of just 830 locations around the globe to feature on Unesco's prestigious list of World Heritage Sites.
A monitoring mission of Unesco experts is also viewing the Houses of Parliament, Westminster Abbey and Liverpool's waterfront to assess threats to their settings from modern development plans.
The UK visit arises from a decision at the July summit of Unesco's World Heritage Committee in Vilnius, Lithuania, voicing "great concern" over proposed new developments which "appear not to respect the significance" of the Tower or its setting.
London's planning policies to protect the Tower "do not seem to be applied effectively" and there is a danger that statutory protection for views to and from the Norman castle built by William the Conqueror could be diminished, said the committee.
Unesco has called on the UK to provide a report on its efforts to protect the Tower by the end of February 2007 and warned it could be placed on the Heritage in Danger list.
World Heritage Committee member Kishore Rao told BBC Radio 4's Today programme: "If you have modern buildings coming up all around it and people - pedestrians - can't view a certain historic building against the skyline or in relation to other historic buildings in the vicinity, then it loses the value.
"You don't only maintain the authenticity of the site - that means that the structure itself is not damaged or modified - but also that it is presented in its historical context."
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Neville Shulman, of Unesco's UK Culture Committee, told the programme: "Once you lose the views, you start to lose the intrinsic value of the buildings and suddenly people start to think about demolishing, altering, changing in many different ways. I think we have got to protect it in all ways."
