Santander is facing fury from tenants and housing groups for a clause in its buy-to-let mortgage contracts requiring landlords to raise rents by “as much as can be reasonably achieved”.

The wording has been buried in Santander's contracts since 2011. It has come to light only now, however, as buy-to-let investors face an unprecedented squeeze on their incomes due to tax changes applying from April.

Many borrowers are scrutinising mortgage documentation to assess their likely position if they cannot meet repayments.

One private landlord who spotted the clause and brought it to the attention of industry magazine Mortgage Strategy, where it was first reported, said she was "outraged" to discover it.