Buy-to-let mortgage sales fell 26.2 per cent or £1.04bn in March month-on-month, according to Equifax Touchstone.
The firm says this is despite an expected last minute rise ahead of stamp duty rate increases which took effect on 1 April 2016.
Equifax says the average value of a residential mortgage in March was £190,091 (2015: £179,187), and £157,819 for buy-to-let (2015: £151,753).
Residential sales were up 1.4 per cent on February to £12.95bn, the highest month sales figures since the 2008 market crash.
Combined, residential and buy-to-let sales for the intermediated market fell by 5.1 per cent or £855.7m on the previous month.
Scotland was the only region to increase its mortgage sales in March while Northern Ireland saw the steepest fall, down by almost 20 per cent.
London saw a drop of almost 10 per cent in mortgage sales on February.
Equifax Touchstone relationship manager Iain Hill says: “Recent buy-to-let mortgage flows indicate that borrowers took the advice of their lenders, and initiated transactions in good time to avoid an eleventh-hour panic.
“The big question from here is, to what extent will the new stamp duty rates discourage investors from entering into new deals? With so much economic uncertainty, property remains an attractive investment option for many people.
“Given the rollercoaster first quarter of 2016, it will be interesting to see where sales trends go from here.”