The Council of Mortgage Lenders (CML) said 4,200 loans were handed out in April to those purchasing buy-to-let properties - marking a 85.4% fall compared with the 28,700 loans advanced for this purpose in March.
These loans were collectively worth £600 million in April - shrinking back by 86% compared with the £4.3 billion-worth of loans handed out in March for buy-to-let house purchase.
Buy-to-let lending for house purchase was also running at around half the levels seen in April 2015.
On April 1, a three percentage point stamp duty increase was introduced in England, Wales and Northern Ireland for people buying second homes, including buy-to-let investors. Stamp duty has been abolished in Scotland but a similar tax hike took place there to mirror the changes in the rest of the UK.
The CML said the slowdown in lending in April had been expected, as the stamp duty increase caused many property purchases to be brought forward into March, as landlords rushed to beat the deadline.
Its figures also showed first-time buyers borrowed £3.9 billion for house purchase in April, a figure which was down by 11% compared with March but still up by 15% compared with April 2015.
Home movers borrowed £4.3 billion for house purchase in April, down 53% on March and down 14% compared with a year earlier.
Paul Smee, director general of the CML, said: "There is a sense of calm after the storm this month, as lending eased back, following the significant rises in activity in March as borrowers looked to beat the second property stamp duty deadline. We expect the market to take several months to return to its previous levels after the lending surge."
Meanwhile, with mortgage rates remaining attractively low, remortgage activity bucked the general downward trend. Some £6 billion-worth of remortgage loans were handed out to home owners in April - a 25% increase compared with March and a 40% increase compared with a year earlier.
Howard Archer, chief UK and European economist at IHS Global Insight, said: "The strong suspicion is that housing market activity will be pressurised in the immediate term by a combination of weakened interest from the buy-to-let and second home sectors as well as heightened concerns and uncertainties over the UK economic outlook, particularly in the run-up to June's referendum on EU membership."