The eight-letter word came top due to the regular placing of an 'E' where the first 'A' sits.
Second in the list was 'definitely', which often falls victim to a string of mistakes including mixing up the second 'I' with an 'A'. Another common error is dropping the final 'E'.
'Manoeuvre', which is problematic due to the unusual combination of OE and U, came third and 'embarrass', in which an R or an S often falls by the wayside, was fourth.
'Occurrence' emerged as the fifth most commonly misspelt word due to confusion over the double C and double R.
A spokesman for market research company www.OnePoll.com, which carried out the study of 3,500 Britons, said: ''There seem to be some words which we always struggle to get down onto paper, and 'separate' is one of those which eludes us.
''A common mistake many make is writing a word the way it sounds which leaves us muddling up one letter with another and getting it wrong.
''Fortunately, computers' spell-check corrects wrongly spelt words for us, but that means we become lazy and never learn the correct spelling.
''There's no excuse not to learn how words are formed - it's drilled into us from such a young age and if the words are frequently used we should make a conscious effort to get it right next time.
''The fact we judge other people's intelligence by their written word, yet don't like to be judged ourselves, means we should all pick up a dictionary once in a while.''
The study also found 'consensus' and 'unnecessary' cause problems for many, coming sixth and seventh.
'Acceptable', which causes issues partly because of the two Cs, was eighth, while 'broccoli' came ninth.
The top ten was completed by the word 'referred'.
Other difficult words to feature include 'bureaucracy' at 11, 'connoisseur' at 14 and 'particularly' at 17.
Confusion over the number of Ns in 'questionnaire' meant it came 13th, with 'entrepreneur' 16th and 'parallel' at 20.
Words which just failed to make the top 20 include 'calendar', 'pigeon' and 'changeable'.
It also emerged one in six people often spell words so incorrectly while typing their PC doesn't recognise the word they are attempting.
Two out of three admitted using spell-check on computers had made them lazy when writing letters or notes by hand.
One in five blamed predictive text messaging for their bad spelling.
But despite the frequency of errors, most of us (77 per cent) believe our spelling is either 'good' or 'very good'.
The study also found 46 per cent judge other people on their spelling, with 27 per cent admitting they believe people who cannot spell are 'thick'.
Three out of ten said they were embarrassed by their poor spelling skills and one in ten corrected others when they spell something incorrectly.
One in five said it was their belief the art of spelling was something 'you just learn in school'.
Top 20 misspelt words:
1. Separate
2. Definitely
3. Manoeuvre
4. Embarrass
5. Occurrence
6. Consensus
7. Unnecessary
8. Acceptable
9. Broccoli
10. Referred
11. Bureaucracy
12. Supersede
13. Questionnaire
14. Connoisseur
15. A lot
16. Entrepreneur
17. Particularly
18. Liquify
19. Conscience
20. Parallel