A YouGov survey for Movember asked men to say how many friends, if any, outside the home they would discuss a serious topic such as worries about money, work or health with.
Just over half (51 per cent) said two or fewer but one in eight overall said none.
That equates to around 2.5 million men across the UK.
When the results are broken down by age it suggests men have fewer close friendships as they get older, with only seven per cent of those under 24 saying there were no friends with whom they would discuss a serious topic but 19 per cent of over-55s.
And while marriage offers lifelong support and companionship, the study shows that married men have some of the lowest levels of support outside the home.
They are more than a third more likely than their single counterparts to say they have no-one to turn to outside of the home.
While 11 per cent of single men said they had no friends to turn to in a serious situation, that rose to 15 per cent among married men.