More than one in five charities are failing to communicate their vision to the wider public, according to not-for-profit think tank nfpSynergy.
Its Mission Impossible research looked at 80 of the UK's largest charities' websites to assess how an outsider might perceive their mission, vision, purpose, values, beliefs and straplines.
The study found that a quarter of charities failed to showcase their strapline, which nfpSynergy defines as the charity's essence.
"It is staggering that a significant proportion of even the larger UK charities still lack any effective statement of their essence," said Joe Saxton, co-founder of nfpSynergy.
VSO's 'Sharing skills, changing lives' and World Vision's 'Poverty, together we can end it' were identified as the two most inspiring straplines.
The study also found that the word 'support' featured most commonly on websites, followed by 'respect', 'partnership' and 'local'.
Gillian Egan, account director at charity communications agency Burnett Works, said: "It is vital that there should be a statement that clearly explains what the charity is about. Competition between charities is getting hotter as they try to mark out their territories."