Technology has left many people unable to spell words such as definitely and separate, a survey has found.
It suggests that the UK has produced an "auto-correct generation" that relies on computer spell checks.
The poll, which questioned more than 2,000 adults, found that around a third could not spell "definitely", while a similar proportion failed to pick the right spelling of "separate".
And around two thirds (65%) picked a wrong spelling for "necessary" from a list that did not include the right spelling.
But while many adults struggle to spell these common words, the poll also found that three quarters of those questioned (76%) think they are good at spelling, with 96% saying that spelling is important.
And many people are relying on spell checks - 18% said they use this all the time, while a further 21% said they rely on it most of the time.
Fewer than one in 10 (9%) said they never use a spell check.
The survey was commissioned to mark Mencap's Spellathon Championships, which take place this week.
Mencap chief executive Mark Goldring said: "With over two thirds of Britons now having to rely on spell check, we are heading towards an auto-correct generation.
"This survey has highlighted that many Britons have a false impression about their spelling ability.