A tax subsidy for individuals with mortgages. 10% of the interest on the first #30,000 of a mortgage is eligible for tax relief. For example, if a mortgage of #30,000 incurred #3,500 of interest in the year, the individual paying the mortgage would be able to claim tax relief of #350 (3,500 W 10%) for the year. This tax relief is taken by means of paying a lower amount to the mortgage lender. In the above example, the individual would actually pay only #3,150 (3,500 - 350) to the mortgage lender over the year. Hence the phrase, relief at source.
Mortgage Interest Relief at Source, or MIRAS, was a scheme introduced by the government of the United Kingdom in 1983 in an effort to facilitate a greater level of borrowing for house purchases; it allowed borrowers tax relief for interest payments on their mortgage.