HMRC has reported a sharp fall in the number of landlords making disclosures under the Let Property Campaign in the last tax year.
The tax yield from the Campaign, which launched in 2013, fell by almost 50% from £34m in 2019/20 to £17m in 2020/21.
Figures released by the authority also confirmed that the number of penalty notices issued to buy to let landlords fell from 8,116 in 2019/20 to only 4,297 notices in 2020/21.
An HMRC spokesperson said "HMRC believes that its customers want to pay the right amount of tax and wants to help those that are not paying the correct amount to put that right.
"The Let Property Campaign is an opportunity for landlords who owe tax through letting out residential property, in the UK or abroad, to get up to date with their tax affairs in a simple, straightforward way and take advantage of the best possible terms.
"If you're a landlord and you've undisclosed income you must tell HMRC about any unpaid tax and you'll have 90 days to calculate and pay what you owe."
When the Campaign was launched by HMRC seven years ago, it estimated that up to 1.5m landlords had underpaid or failed to pay tax, and urged voluntary disclosure or the risk of bigger penalties.
However, with a tax yield to date of only £190m, HMRC has secured only 38% of its original target of £500m.