Only 15 per cent of people could remember the date.
The same study found even fewer people could remember details of the Hillsborough disaster.
When Dr Dan Wright and colleagues from the London School of Economics surveyed more than 2,000 people they found that only 11 per cent had a good memory of Baroness Thatcher's resignation in November 1990, and only 9 per cent of Hillsborough in April 1989.
In their paper given to the conference yesterday they found, however, that those more likely to remember Lady Thatcher's departure were in the professional and managerial classes.
'We were rather surprised that such a low number had a vivid memory of her resignation because another study among students showed 86 per cent were able to recall the events well,' Dr Wright said. But they had been given detailed questions which jogged their memories and more time to answer.
Their study, funded by the Economic and Social Science Research Council, aims to assist people who design questionnaires because it is known that long-term memory of events is faulty. 'What we saw were clear social differences. The highest social classes saw the event as more emotional and more important and had clear responses to the questions. But memory steadly declined after social classes I and II.'
In their paper they say: 'Although Thatcher's resignation was clearly important, it is worth pointing out that the Gulf war followed shortly afterwards and, therefore, many people may not consider her resignation to be the most important event of that time period.'
Dr Wright said yesterday: 'She was certainly a central political figure and although she said she would go on there was speculation . . . that her days were numbered. Kennedy's death was a huge shock. It may be that shock is important in helping people to fix the event.'