The survey, published today, was commissioned by Mencap, a UK based charity, to mark this week's launch of its Spellathon competition, where children and adults are encouraged to test their spelling.
Only one in five adults out of 2,000 who took a short spelling test were able to answer all five questions correctly, the 'Daily Mail' reported.
Sixty-five per cent failed to spell 'necessary' correctly while 33 per cent struggled with 'definitely' and 'separate'.
Respondents also admitted to extensive use of computer spell checks and auto-correct functions while writing emails and documents.
Eighteen per cent said they used spell checks all the time, while a further 21 per cent relied on them most of the time.
Despite this, Britons rate themselves as competent spellers, with 76 per cent describing their abilities as "very good" or "fairly good".
Almost all those questioned believed spelling was important.
Students emerged as the worst spellers, with only 13 per cent getting all five questions in the test correct, against 21 per cent overall.
Women were better at spelling than men, with women aged 65 and over scoring the highest marks.
Mark Goldring, chief executive of the learning disabilities charity, 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. Today's tough economic climate means that poor spelling on a CV is fatal, as it says that an individual cannot produce work to a given standard, no matter how highly qualified they might be," he said.
"Language used by a company or person is a reflection of their attitude, capabilities and skill," he added.
"The failure to test spelling, as well as grammar and punctuation, in exams over the past decade has been a costly one," UK Schools Minister Nick Gibb said.
