Great Ormond Street Hospital Children’s Charity saw its income fall by almost £16m last year.
The charity’s accounts to 31 March 2019 show that its income fell from to £87.5m, down from £103.2m in the previous year.
But spending increased only from £68.9m to £71.3m, meaning the charity still finished the year with a significant surplus.
Donations fell from £67.6m to £54.7m, the accounts show, with legacy income also falling by £3.3m to £23m in 2018/19.
This meant that despite a £1m increase in trading income, total fundraising income at the charity fell from £99.4m to £84.2m.
In the accounts, the charity says there were two reasons for the fall in income: "First, the 2017/18 legacy income included a one-off £8.6m related to the change in the estimation process of our legacy accrual", which meant legacy income in 2018/19 was £7.2m better than the underlying legacy income in 2017/18.
The accounts also say: "Second, the £12.9m reduction in donations from 2017/18 was primarily driven by delays in the construction of the Zayed Centre for Research and £10m in related donor instalment payments (linked to construction milestones). These monies will be received in 2019/20."
The Zayed Centre for Research will be a research facility for rare diseases affecting children and is due to open next year.
Work on the centre cost £27.1m in 2018/19, compared with £27.6m the year before.
A spokeswoman for Gosh Children’s Charity said: "As part of our normal fundraising cycle we expect to see variations in our overall income. Our 2017/18 reported results were favourably affected by a ‘one-off’ change in the basis for reporting our legacy income, which contributed an additional £8m to our results.
"Our 2018/19 results were adversely affected by the timing of a significant donation, which has now been recognised in 2019/20."
The charity supported more than 80 research projects in 2018/19 with funding of £6.9m, the accounts say.
An average of 619 children from across the country come into Gosh every day, the accounts add.