Income at the conservation charity WWF-UK fell last year by 13 per cent, or about £10m, the charity’s latest accounts show.
The accounts say that income was £60.8m in the year to 30 June 2017, down from £71.1m the previous year.
The fall in income was partly due to a £4.2m reduction in legacy income after the charity received a £4.8m gift in the previous year, pushing income to record levels.
But income from government grants and corporate partnerships also fell on the previous year, by £2.1m and £2.3m respectively, the accounts say.
A statement from the charity said: "Under challenging circumstances for all charities, our income for 2016/17 remained steady with 2014/15. Income fell compared with 2015/16 as anticipated, and this was principally due to an exceptional legacy gift we received during that financial year."
Expenditure was £50.9m, a slight increase from £49.3m the previous year, according to the accounts.
In his introduction to the latest accounts, Sir Andrew Cahn, chair of the WWF, says the charity "will need a revolution in the level of our funding if we’re to achieve the step-change in our impact that we’re determined to accomplish".