Macmillan Cancer Support has said that a change in the way it records public complaints led to them increasing by almost a half last year.
The charity's annual accounts, published this week, show that its income in 2017 increased to a record high of £252.7m in 2017, up from £247.4m in the previous year, due in part to a £2.8m increase in fundraising income to £247.7m.
It also received its two largest-ever legacies in 2017, one worth £7.3m, the other worth £6.3m.
The accounts also show that Macmillan received 6,600 complaints about its fundraising, marketing and communications, an increase of 2,000 on the previous year.
The charity says in the accounts that the increase reflects a new approach to feedback in which employees "capture all individual comments about how we can improve, not just complaints".
The accounts say that this approach has been adopted to help Macmillan "learn more lessons, put more things right and consistently deliver what our customers expect".
A spokeswoman for the charity told Third Sector: "At Macmillan we take complaints very seriously. We rely entirely on generous donations from members of the public and work hard to maintain their trust. We actively encourage people to tell us about their experiences of Macmillan, so we can continue to be the most trusted charity in the UK.
"In 2016 we updated our definition of complaints, to 'any expression of dissatisfaction', meaning that more customer feedback has been recorded as complaints, particularly in the area of social media.
"In addition, we had an issue with one of our suppliers to do with a particular area of donations, which resulted in increased complaints in 2017. We addressed this as soon as the issue was highlighted."
Expenditure increased from £245.6m to £256.4m, but a net gain on fixed and current asset investments of £5m meant that Macmillan made a £1.3m surplus, according to the accounts.