Specialist lender Paragon Mortgages has issued a warning to the buy-to-let marketplace that stringent lending criteria is essential for inexperienced private investors.

"We do not allow new borrowers to be over-reliant on rental income to cover the mortgage on their first investment property and we are becoming concerned that the criteria adopted by a number of new lenders in this specialist market are clearly designed to underwrite the property not the individual looking to make a relatively-low risk investment," said Paragon Mortgage's Mr John Heron.

Volumes have been doubling in the buy-to-let market each year since 1997 the lender says. Demand is concentrated at the cheaper end of the sector.