The asbestos is thought to originate in concrete noise insulation panels which were installed in sections of tunnel wall on the Central, Northern and Jubilee lines about 30 years ago and have since been largely removed.
The company said yesterday that extensive air monitoring tests had been carried out by the scientific services department after small pieces of asbestos were found near Bethnal Green, east London, in early July. It was satisfied that there was no danger to passengers or staff.
'We are not talking about sackfuls of the stuff, we are talking about four or five pieces which have been found so far,' a spokesman said. 'But we are taking it seriously and continuing tests to see if there is any more of it about.'
The main rail union, RMT, yesterday called for medical checks for staff who have worked at affected sites. Jimmy Knapp, general secretary, said: 'We will be demanding an immediate inquiry to establish precisely how this material came to be in the ballast.'
The pieces of asbestos were found buried in track ballast during a track cleaning programme which has since been suspended.
The Underground spokesman said that the work was carried out by an outside contractor whose employees were wearing protective clothing.
He said that the asbestos probably fell on the track when the panels were installed.
The discovery prompted a memo from Robert Williams, a night maintenance manager, who warned that 'if there are any large amounts of asbestos present then there could be some danger'.
The Health and Safety Executive said yesterday that it understood dust levels to be well below exposure limits. 'The Railway Inspectorate is aware of the problem,' a spokesman said.