The taxman is preparing to clamp down on tens of thousands of buy-to-let property owners who may not have paid enough tax, The Times has learnt.

HM Revenue & Customs has identified 80,000 landlords who may have claimed too much tax relief or have failed to declare the amount of rent they receive from the property, or a capital gain made on the sale of the property.

The Revenue can claw back unpaid tax from as far back as six years, which means that some of those who have bought properties to rent or are letting their own home could face tax bills so large that they may have to sell their property.

It also has the power to impose penalties, which can reach the same value as the unpaid tax bill, and charge interest on the sum.

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The campaign represents the latest attempt to extract as much tax as possible to boost revenue for the Exchequer. Britain will have the biggest budget deficit in Western Europe next year.

As part of a campaign to tighten up the buy-to-let market, a group of “ghost” landlords who have failed to declare themselves as property owners will also be targeted.

The campaign will also draw on information from banks and tenants and from letting adverts.

It is understood that many of the 80,000 taxpayers who have underdeclared have incorrectly claimed deductions for mortgage repayments. Landlords have to pay income tax on the rents they receive from tenants. But under the law, landlords can offset some of the tax they pay if they have an interest-only mortgage.

For example, a landlord who pays 40 per cent tax on a rental income, can offset the total £625 monthly cost of a £150,000 interest-only loan against the tax bill.

However, the repayments must relate to the interest part of the mortgage alone. Landlords cannot claim deductions for mortgage repayments that pay off some of the capital borrowed. A number of mortgages have an element of capital repayment, of which the borrower may be unaware.

The ease of obtaining buy-to-let mortgages, coupled with buoyant house prices, has led to a boom in the investment property market over the past decade. There are more than 400,000 buy-to-let landlords in Britain. They borrowed more than £94 billion in 2006.

Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants, has urged the Revenue to use a “light touch” in its campaign.

“Buy-to-let investors are generally not tax evaders. Many think the mortgage interest is at such a level that it covers the rental income and that they don’t have any additional tax to pay. But the tax situation is so complex they may well have tax to pay,” Mr Roy-Chowdhury said.

The taxman will alert landlords to potential discrepancies in their records initially by sending them a leaflet on rental income and capital gains tax.

Recent figures from the Council of Mortgage Lenders state that outstanding buy-to-let mortgages alone now total £9.5 billion.

Mike Warburton, senior tax partner at Grant Thornton, the accountant, said that landlords could take advantage of the Revenue’s reduced-penalties scheme currently on offer for those who have failed to declare their offshore income.

The Revenue is offering a 90 per cent discount on the usual penalties that apply if individuals confess to unpaid tax on offshore income before June 22.

“This is your chance to come clean,” Mr Warburton said.

Landlords in Britain face a capital gains tax bill of more than £4.1 billion, with landlords facing an average bill of £48,600 based on 2006 housing prices, according to figures from Landlord Mortgages, a specialist buy-to-let mortgage broker.

The average house price in 2006 according to Halifax, Britain’s largest mortgage lender, was £179,601.

A spokeswoman for the Revenue said: “We met representatives of the accountancy profession this week for their views on how we can best inform landlords of the obligation to report their property income to us.”

The Revenue has become much more aggressive about collecting the tax in the past few years.

Measures introduced include a daily penalty of £60 a day for those who persistently fail to file their self-assessment returns on time, the removal of the cap on penalties it could charge to the value of the unpaid bill and bonuses for inspectors who chase debts.

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