Those with more than 1.23 acres of land face having to pay a 28 per cent levy on gains on the sale of a home. The new stance by tax authorities, reviving the enforcement of rules on the statute book since the 1960s, threatens to upset the long-held assumptions of many that profits from the sale of one’s main home are exempt from tax.

Accountants reportedly said the number of inquiries from HM Revenue and Customs about tax due on gardens had gone up from a couple a year to three a month.

It is feared that the Government could be moving towards a cap on tax-free gains as in America, where only $250,000 of any profit is exempt and homeowners have to prove they have lived there for at least two years.

The emergence of the garden tax clampdown comes ahead of George Osborne’s autumn statement this week. He has come under pressure from Liberal Democrats to bring in a mansion tax on high-value property.

Currently, householders can sell their main residence and grounds free from Capital Gains Tax if the garden is less than half a hectare (1.23 acres).

Even if the grounds are bigger, they can claim full relief if it can be shown that they are “required for the reasonable enjoyment of the house as a residence”.

HMRC denied that it had “launched a campaign to look at private residence relief” but admitted it was checking on transactions using a computer programme called Connect, which enables it to cross-check stamp duty records paid by homebuyers with CGT declarations by sellers.

Gary Heynes, tax partner at Baker Tilly, told the Sunday Times: “Our clients have had to demonstrate the grounds they are claiming for are an integral part of the property with no hedges or streams that separate them from the residence.”

In an example of the application of the rules, a homeowner selling a £2.3 million house in Kent with eight acres might find that HMRC only allows three to be sold free from CGT. If the other five acres were worth £180,000 of the total price, compared with a value 10 years ago of £60,000, the seller would have to pay tax at 28 per cent on the £120,000 gain – £33,600