It appears that there could be over 150,000 people affected by the change in rules, who may not be aware at present that it even affects them.
The way that HMRC operates the upcoming reduction in tax relief means that people who are currently within the basic rate may suddenly find themselves into the higher rate and losing mortgage interest relief.
To calculate the tax position it is a three stage process:
1) All mortgage interest is disallowed from the computation.
2) Then the check is made to see if someone is basic rate or higher rate.
3) Finally a credit is given for mortgage interest, but if someone has become higher rate due to stage 1 then they could find that they lose out and were unaware beforehand as they thought they were a basic rate taxpayer.