File photo: Commitment was much more unlikely among cohabiting couples than among married people, it found.

Nearly one in six people in unmarried relationships admit they are worried about their partner’s loyalty, according to a major survey.

It found couples valued communication, trust and commitment as the pillars of a relationship – but 15 percent of unmarried people fear their partner is less committed than they are.

The level of doubt and mistrust among unmarried couples is two-and-a-half times that of those who are married.

The study, for Relate, Relationships Scotland and Marriage Care, suggests there are differences between married and cohabiting couples.

The YouGov survey of 5 000 people found more than two-thirds said trust was important, 52 percent pointed to communication and 37 percent said commitment. This compared with just 19 percent who thought fidelity was the most important factor, while only 13 percent cited a good sex life.

But commitment was much more unlikely among cohabiting couples than among married people, it found.

Measuring what it described as "commitment asymmetry" among couples, the study found 15 percent of those who were unmarried admitted concern that their partner’s level of commitment was different to theirs.

Among married couples, however, only six percent had similar concerns.Harry Benson, of the Marriage Foundation think-tank, said: "This is one of the better explanations for why married couples are more likely to stay together – because they have had a clear conversation about their future and their commitment is mutual and explicit. It is therefore no longer tenable to claim marriage and cohabitation amount to much the same thing."

But Relate’s Chris Sherwood said: "We absolutely celebrate marriage for those who choose it, though we don’t think it’s the only way to have a loving and committed relationship. In our experience and as evidenced by a wealth of research, it’s the quality of a relationship that matters through life’s ups and downs rather than its status."