Giles Pegram, the charity's director of fundraising, told Third Sector it was becoming more difficult to recruit new donors and to upgrade existing supporters.
There was also a slight increase in direct debit cancellations, he said, but the most significant loss was £2m of legacy income, mainly because of falling house prices.
He said the charity had come through previous recessions relatively unscathed. "But there is something fundamentally different about what we're going through at the moment," he said.
"We're still forecasting that we're going to be on budget this year, but we know we're going to struggle. We started looking at support and overhead costs, but we are having to go deeper."
Joe Mann, deputy general secretary at the trade union Community, said it was in discussions with the charity about the cuts. "We will represent our members, but we don't want a battle," he said.
Oxfam, which is planning to cuts costs by 15 per cent over three years (Third Sector, 13 August), said it might cut up to 45 jobs and has begun a staff consultation on redundancies.