The Bank's governor, Mark Carney, said he was concerned about high levels of lending to landlords and that the Bank would take action.
"There are a number of things happening ... we are watching it closely and we will take action," he told the FT.
Mr Carney said the problem was that investors might sell their properties at the same time if house prices fell.
In September, the Bank's Financial Policy Committee (FPC) made a similar warning about the buy-to-let market.
The committee, which is led by Mr Carney, said the growing market posed a threat to the UK's financial stability.