Worried your boyfriend is never going to propose? Then buy him a calculator.

Mathematicians have come up with a 'fiancee formula' that allows men to work out the perfect time to pop the question.

All he needs is the age he would first consider marrying and his cut-off point - and the equation does the rest.

Maths professor Anthony Dooley said: 'Applying maths to matters of the heart is always dangerous. In life you are dealing with emotions and have to think much harder.

'But if you want to work out the right moment to start getting serious, this gives you a mathematical framework.'

The formula is based on a statistical technique known as optimal stopping - or the best time to do something.

Professor Dooley's colleague, Professor Bruce Brown, said the formula was a 'reasonable approach' unwittingly followed by many young men, including himself.

Writing in the third person, he said: 'As for the author, he can tell you that, looking back and doing some calculations, he did follow the marriage solution, albeit by accident, and it has worked out perfectly.'

The formula was devised with men in mind but could equally apply to women, including those uncertain about whether to accept a proposal. It could also help nervous men calculate when to avoid the ultimate commitment.