Despite the 10% drop, the BSA said October gross lending was broadly in line with the average of £2bn recorded over the previous three months.
Mutuals’ net lending was -£290m in October compared to -£11m in September.
Mortgage approvals by mutuals fell 8% in October to £1.76bn from £1.912bn in September.
Adrian Coles, director-general of the BSA, said: “Lending activity across the market has shown signs of weakening in recent months. Although lending by mutuals fell in October, gross lending remains similar to the average over the preceding three months.
“Reports of falling house prices and government spending cuts have lowered consumer confidence which has put further downward pressure on demand for mortgages.”
However, Coles added: “An encouraging trend in recent quarters has been the growth in mortgage approvals at mutuals and this may lead to some recovery in mutuals’ market share over the next few months.”