In the final quarter of 2014 the proportion stood at 62%, with the rest accounting for new purchases.
Average loan to values have also risen in three month period from 63% in the final quarter of 2014 to 66% in the first quarter of 2015.
David Whittaker, managing director of Mortgages for Business, said: “A great deal agreed last year may be uncompetitive by today’s standards. So this stampede is completely rational – it represents a charge by landlords to make the most of an unprecedented economic situation.
“Remortgaging is often done for the purposes of raising extra capital, and this is clearly reflected in higher loan to value ratios. However, this is by no means an unwelcome trend – and could in turn open the door to more new purchases and investment by landlords.
“Rental yields are healthy and there is a gathering demand from an increasingly prosperous base of tenants. So the fundamentals of the rental market – and of landlords’ finances – are still extremely solid.”
For houses in multiple occupation (HMOs) remortgaging is now an even higher proportion compared to loans for home purchase, standing at 73% in the first quarter of 2015, up from 70% in the fourth quarter of 2014.
And average loan to values have risen from 64% in the fourth quarter of 2014 to 70% in the first quarter of 2015.
The same trend more pronounced still for multi-unit freehold blocks, (MUFBs) with remortgaging representing 89% of mortgages in quarter one 2015, compared to just 42% in the final quarter of 2014.
Semi-commercial property witnessed the same trend but with a more gradual change, from 86% to 87% of new loans agreed for remortgaging.
Whittaker added: “Landlords are reporting a buoyant rental market, driven in large part by a resurgent jobs market – and now even more encouraging signs on wages.
“In turn, this will stimulate many landlords to invest further although one major hold-up in an otherwise sunny outlook is a long shadow of political uncertainty.
“This is only partly about specific policies. For example rent controls could be a well-intentioned but disastrous blow to the industry. However, more of an immediate worry is the far more general risk of a power vacuum after an election barely three weeks away, the associated effect on the financial markets – and ultimately on mortgage rates.”
