Mr Justice Ewbank said that the London-based firm, Dawson and Co, failed to follow Sigrid Griffiths' instruction to hold up the divorce until a financial settlement had been reached.
Mrs Griffiths, of Victoria, central London, claimed that her solicitors' failings had cost her pounds 74,000 in lost pension and legal bills.
After the ruling, she said she had been 'absolutely pole-axed and flabbergasted' when the decree absolute went through in June 1985.
'Most women in my situation do not know the law so they go to a solicitor and expect that which is possible will be done for them. If it is not, it should be taken up.'
She said she had nothing to celebrate. 'My life changed totally when I lost my husband and there's never any compensation for that.'
After the divorce, Mrs Griffiths received maintenance of pounds 10,000 a year, which was halved by consent to pounds 5,000 in 1990. Since Sir Eldon's retirement from Parliament last year - he was MP for Bury St Edmunds for 28 years - she had received a nominal 5p a year.
She told the High Court that as a result of Dawson's neligence, she had lost the right to the pension she would have received as an MP's widow in the event of Sir Eldon's death.
Kenneth Wood, a senior partner of Dawson and Co - which faces costs estimated at pounds 25,000 - said the firm was considering an appeal.
Giving judgment yesterday, Mr Justice Ewbank said a competent solicitor should hold up a decree absolute while financial matters, including pension rights, are studied. The solicitor should also look into the rules of the relevant pension fund.
David Robson, who handled the divorce before he retired from Dawson and Co, did neither. He took the 'surprising' view that an application to hold up the divorce was an 'extortion' and would be 'unsporting'. Mr Justice Ewbank said it was an 'injustice' that most pension funds made no provision for a divorced wife to have a widow's pension.
Last night, members of the Solicitors' Family Law Association, conceded that it was difficult for clients to check on the quality of their lawyers' work. However, they said that solicitors should always take the time to explain 'the important areas of the work they are doing', as well as sending clients copies of all relevant correspondance.
If clients are not kept informed, they should talk to their solicitors and then, if they are still dissatisfied, use the internal complaints procedures that all firms are obliged to establish. They also have the right to go to the Solicitors' Complaints Bureau.Reuse content