The mortality gap between rich and poor widened further during the early years of Tony Blair's first term, according to research published yesterday by the office for national statistics into inequalities in lifespan.

In 1997-99 the life expectancy of a boy born into a manual worker's home was 1.2 years longer than it had been in 1992-96. This compared with an extra 1.5 years for the child of non-manual parents of the professional, managerial and clerical classes.

The ONS said average life expectancy for male non-manual workers in 1997-99 was 77.3 years, compared with 73.8 for male manual workers. Women from the non-manual classes could expect to live 81.4 years, compared with 78.6 for their manual contemporaries.

In 1997-99 life expectancy for males in non-manual classes was 3.5 years longer than in manual classes. The difference in 1977-81 was 2.6 years. For women the difference was 2.8 years in 1997-99, up from 2.4 years in 1977-81.

The statisticians gave some comfort for ministers trying to argue that Labour started tackling the poverty that feeds through into ill health and premature death.

In 1997-99 life expectancy among unskilled men in the lowest social class rose faster than among the top professional class. But looking across the span of all the classes, inequality increased.

· The ONS said in its quarterly volume of health statistics that cot deaths in England and Wales have fallen to a record low of 231 in 2001, compared with 245 in 2000.

It attributed the drop to increasing awareness among parents of advice on the best sleeping arrangements for babies. However, the Foundation for the Study of Infant Deaths said it was concerned about the increasing number of deaths registered as "unascertained" and not included in the figures.