Fresh claims that the Government intends to use sophisticated “spies in the sky” to detect people’s home improvements or amenities that would affect the value of their properties were made yesterday.

The Conservatives, who have been carrying out an investigation into the council tax revaluation database, say that Northern Ireland is being used as a testing ground for the new technology, involving aerial photographs and satellites.

Caroline Spelman, the Conservatives’ local government spokesman, said that in Northern Ireland, where a revaluation has just taken place, 21st-century technology was helping to locate new conservatories, garages, home extensions or scenic views, and to identify attractive neighbourhoods.

Ministers have already ruled out a council tax revaluation in this Parliament and yesterday denied the Tory claims that Northern Ireland showed that it was happening by stealth.

Phil Woolas, the Local Government Minister, described the Tory claims as “irresponsible scaremongering”. He added: “We have made clear many times that Northern Ireland is not a testing ground for England and their system is completely different. There is no revaluation by stealth going on and a revaluation will not take place in this Parliament.”

With the report on local government finance by Sir Michael Lyons being published this week, the Tories said that, contrary to ministerial claims, the Valuation & Lands Agency, which acts as the Northern Ireland council tax inspector, has teamed up with Ordnance Survey to use “oblique aerial photography” and “large-scale mapping” for tax valuations on people’s homes. The two bodies have been combined into one “shadow agency” to facilitate data sharing between them.

The technology is said to be accurate enough to pick out garden features, parking spaces and extensions. It can also calculate each home’s local tax bill by taking into account not just the size and features of the home, but also its location.

Ms Spelman said: “Labour ministers may claim to have postponed the council tax revaluation, but in reality it is well under way by stealth, with Northern Ireland being used as the testing ground for new technology. A computer database is logging the precise characteristics of every home — including the number of bedrooms, floor space, and features like patios, gardens and parking spaces.

“The public have already expressed concern at the prospect of tax inspectors with clipboards and cameras forcibly entering their homes. Now it appears that the Government will also be using planes and satellites to snoop on people’s homes and lay the ground for taxes on home improvements and nice neighbourhoods.”

Meanwhile, Ruth Kelly, the Communities Secretary, has ruled out any changes to council tax bands before the next general election. The Times revealed last week that Sir Michael will endorse a new higher band for homes worth more than £1 million when he publishes his report on reforming the system this week.

But Ms Kelly poured cold water on the idea — which would affect more than 80,000 homes, mainly in the South East — insisting it was a “nonrunner” during the current Parliament at least.

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