For the love of Leicester, it's time to cherish our tarnished urban jewels

English Heritage warns that one in seven conservation areas is threatened by change

With its identikit retailers from Laura Ashley to KFC and acres of grey granite paving, Leicester High Street might not be an obvious choice as one of England's architectural jewels. But thanks to the creeping evils of plastic windows and neon lighting it seems this unsung beauty spot – along with 726 others – is at serious risk of losing its urban "charm".

English Heritage issues a dire warning today that a host of the country's most alluring townscapes (and some less desirable vestiges of the nation's built heritage) are being destroyed by satellite dishes, advertising hoardings and neglect.

The conservation watchdog has placed one in seven of England's conservation areas, set up to preserve the "characterful appearance" of the architectural landscape, on its "at risk" register after finding that disrepair and modern tinkering with buildings are inflicting lasting damage on the historic fabric of towns and cities.

A study of the 9,300 conservation areas, designated by local authorities, warns that a plethora of ills created by councils and homeowners, from traffic calming to the demolition of front garden walls, is eroding the integrity of the urban landscape in places from Lancashire mill towns to London's chi-chi neighbourhoods. The march of uPVC glazing is highlighted as the most pernicious scourge, affecting 83 per cent of all areas.

The survey highlights the spread of satellite dishes, scarring 38 per cent of conservation spots such as Baron's Court, west London, where they partially obscure blue plaques commemorating the one-time residency of Mahatma Gandhi and the aviation pioneer Sir Geoffrey de Havilland.

Simon Thurley, the chief executive of English Heritage, said: "When we think about places that are protected we tend to think only of listed buildings. But the reality is that millions of us live in, work in, pass through or visit conservation areas every day. They form the historic character of the nation but unseen over the years, hundreds of thousands of changes have been made where that character has been gradually expunged.

"We need to put that situation into reverse if we want to preserve the quality of life in these places. If we lose these small but important details, such as windows, doors and front gardens, then we lose the character and the history that made these areas special in the first place."

The list of unwanted attributes includes poorly maintained roads, affecting 60 per cent of conservation areas; street clutter such as unnecessary bins, road signs and bollards (45 per cent); loss of front garden walls (43 per cent); the impact of advertisements (23 per cent); and neglected green spaces (18 per cent). Leicester High Street, which has undergone a £19m facelift, was criticised for its "homogenisation" of outlets, loss of original shopfronts and the decay of vacant buildings. Similar criticisms are levelled at Derby city centre and that other architectural masterpiece, Clapham Junction in London.

A spokeswoman for Leicester City Council said any suggestion of urban blight was unfair: "The high street has seen a lot of investment and we would still like to invest more. But this is not about eyesore and buildings in a state of disrepair."

In Thetford, Norfolk, a historic clutch of timber-framed houses and Victorian buildings has been scarred by alteration, extensions and satellite dishes, English Heritage says. In Ashton, Lancashire, a cobbled street has been asphalted, to the annoyance of owners of its Grade II-listed houses.

English Heritage says its survey and campaign against plastic glazing do not amount to a fatwa against modernisation. Dr Thurley said: "This is not some kind of posh person's charter. We recognise these are difficult economic times and we not advocating costly measures. It is far cheaper to repair a Victorian window than it is to rip it out and put in a white plastic one."