Joyce Ramsden, who lives near Crewe in Cheshire, discovered the problem when she claimed her widow's pension from the local authority where her husband had worked.
Mrs Ramsden married her husband, a widower, after he had retired from his job in the education department of Bolton local authority.
They had been married for about four years when he suddenly contracted leukaemia and died.
The pension paid by the authority was only about pounds 10 a week compared with her husband's pension of pounds 144 a week.
When she asked why, she was told that because she married her husband after he retired she was only entitled to a small amount of pension instead of half - pounds 72 a week - which she would have received if the marriage took place before he retired.
The total received by Mrs Ramsden, including the state pension, is about pounds 69 a week. She said: 'My husband retired early to look after his first wife, who died of motor neurone disease. She did not really benefit from his pension.'
The maximum amount that schemes can pay by law to the surviving spouse is two-thirds of the member's pension.
David Philips, technical officer of the Greater Manchester pension fund, which runs the local authority scheme, said: 'Originally it did not recognise post-retirement marriages.'
However, changes in legislation in 1978 meant it had to pay some benefits to the surviving spouse of a post-retirement marriage. 'The benefit would be half that accrued after that date,' he said.
Some occupational schemes were set up this way to prevent death-bed marriages, where the purpose of the union is to pass on the pension benefit.
Other occupational schemes have very different rules. For instance, the mineworkers' pension scheme entitles the widow to two-thirds of the pension regardless of when the marriage took place.
Jon Heathfield, administrator of the mineworkers' scheme, said: 'The legal wife would be entitled to the pension benefits. A previous partner would not have any right to the pension.'
He said the situation would become more complicated if the member was separated but not divorced and was living with someone else.
'Both women would have a right to his pension. It would be up to the trustees to sort it out,' he added.
British Telecom pays out a maximum of half the pension benefits to the surviving spouse, again regardless of when the marriage took place.
Barclays operates the same system for its employees, but said the benefits would only be paid to a legal spouse.
Stephen Ingledew, director of the independent financial adviser Frizzells, said that personal pensions could offer a great deal more flexibility in terms of the widow's benefit.
'When you take out the pension you nominate how much you want to be for the surviving spouse,' he said.
At the moment the pension industry is discussing proposals which would make any pension benefits part of divorce settlement.
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