Climb the hill to London's Alexandra Palace on a clear day and you can see the city's history spread out before you.

London looks like what it is; a series of villages strung together by roads. The spires of Canary Wharf share the skyline with a handful of unexceptional residential blocks. If it is very clear, you can just see the dome of St Paul's Cathedral.

The panorama of terraced homes interspersed with parks and commercial areas is a pleasure. But is it sacrosanct? If anything, the view tells the history of urban sprawl. As the population surged, farm and swamplands were cleared and waterways filled in to make way. The city spread outwards, not upwards.

Is a London skyline unencumbered by tall, modern architecture the best way to preserve the cultural and financial vibrancy of the city? That is the debate that is emerging amid growing pressure from developers and occupiers. Last month, the Department for the Environment, Transportation and the Regions called in the application for a 50-storey tower planned by Heron Group at 99 Bishopsgate, in spite of its having received overwhelming support from the local planning authority, the Corporation of London.

Plans for Europe's tallest tower, - described by its architect, Renzo Piano, as "our shard of glass" - are under consideration for a site near London Bridge Station. The Greater London Authority itself is arguing for a rethink of planning policy to allow tall buildings near transport interchanges. Residential densities, it says, need to be higher in inner-city areas.

Why is London having this debate? After all, tall buildings are a prominent feature of modern - and not so modern - architecture. Why do they engender such intense emotion?

At a conference on tall buildings sponsored by the Royal Institute for British Architecture this week, Mr Piano set out his views: "Cities are great because they are centres of intense life," he said. "What tall buildings do is to increase the density of the city through internal growth."

Lee Polisano, principal at architects Kohn Pedersen Fox Associates and architect of the Heron tower at 99 Bishopsgate, said the density of population gave cities their "buzz" and made them environmentally sustainable. "The forecasts are that 90 per cent of the world's total population growth will take place in cities," he said. "People want to be in these centres of activity."

English Heritage, which opposes the Heron Tower, says there is no economic case for tall buildings in London. "The Manhattan factor has seduced cities all over the world into believing that for commercial success they need very high buildings," said Pam Alexander, chief executive, in a paper presented at the conference. But cities such as London, Paris and Rome were low-rise, "bequeathing complex layers of urban life deeply rooted in history".

English Heritage opposes construction of the Heron Tower because, viewed from Waterloo Bridge, it will seem near to St Paul's Cathedral. The view of the cathedral from the Bridge is not protected by legislation.

However, Ken Livingstone, London's Mayor, said the city's development could not be completely constrained by views of St Paul's and that the current rules were "too restrictive".

"London's population peaked at eight million between the wars," he said. "By 1984 it was 6.5m. Now, it is 7.4m and expected to grow to 8.1m by 2015. But the last time we had eight million people, men walked or cycled to work and the women were in the home." Without the construction of higher-density working and living quarters, London will become impossible to navigate or inhabit, he argues.