Unions believe the present practice, to pay pensions only to widows or widowers, discriminates against women and those in same-sex relationships.
It also puts members of public sector pension schemes in a worse position than their private sector counterparts, whose employers are increasingly prepared to pay benefits to any surviving adult dependents.
About 4.5 million public sector workers and a further 3.6 million pensioners are affected by the schemes' existing rules.
John Monks, general secretary of the TUC, which is co-ordinating the campaign, said yesterday: "It is about time that public sector pension schemes abandoned this absurd bit of outdated morality that fails to recognise the realities of today's relationships. The result of this old-fashioned rule is that thousands of men and women are denied dignity in retirement after the loss of their loved one."
Unions are planning a joint approach to the Official Committee on Occupational Pensions. The TUC's campaign follows surveys that show increasing numbers of men and women cohabit with their partners without marrying.
In 1990, the most recent figures available from the Government, 7 per cent of all people aged 16 to 59 were cohabiting. For men aged 25 to 29 and women aged 20 to 24, the figure was about 15 per cent. Among the divorced and separated, the percentage of cohabitees was at least twice as high.
Many private sector pension schemes already pay pensions to non-married partners, including BT, the AA, Unilever, Tate & Lyle, Massey Ferguson and BP.
However, attempts by public sector unions, including those in local government, the Civil Service and firefighters, to persuade employers to amend scheme rules have had no effect. The consequences of employers' refusal to consider changes can seem brutally unfair.
Michelle Bruce, 27, has discovered that, despite living with her partner, Matthew Parsonson, for seven years and being the mother of his two children, she is not entitled to a pension now that he is dead.
PC Parsonson, a police officer, was killed while on duty, just three weeks after joining the force last year. Ms Bruce remembered: "I was at our home in Clacton when there was a knock on our door quite early in the morning.
"Two local officers came round to tell me that Matthew was dead.
"He was on night duty at Stoke Newington, in London, when there was an emergency call. On the way there, the car was in an accident and he died.
"The first few weeks were a blur. It was only afterwards, when I realised I was going to have to support our children, Lorna and Marc, on my own, that the question struck home."
The police pension scheme pays a pension of about 45 per cent of an officer's salary to a widow or widower if the officer died while on duty. Yet Ms Bruce, classed as a cohabitee, will receive not a penny. Her children, as dependents of PC Parsonson, will receive a total annual allowance worth 47.5 per cent of his salary at the time of his death until they are at least 16.
A lump sum worth twice the officer's annual salary is payable to a widow or a personal representative, who is not defined in the regulations and could be a parent or other relative. Ironically, Ms Bruce will receive a small pension - from ICI, where Mr Parsonson worked for five years before joining the Metropolitan Police.
A recent TUC paper says employers' opposition is based mainly on moral grounds, as well as those of cost. Unions claim the increase in costs will be neglible because many couples who cohabit would have been among those claiming as legitimate widows and widowers a generation ago.
Ms Bruce said: "Matthew was always a strong believer in pensions and protecting his family. We were planning to get married just before Christmas, a few weeks after he actually died.
"Lorna is five and Marc is less than a year old. It won't be possible for me to go to work at least until he is at school. What happened to us seems so unfair. Someone should change the rules, not just for my sake, but to stop the same thing from happening to others."Reuse content