Dr Samuel Kiberu, who acted as a locum consultant at Grantham District hospital and at least five others, also conducted post-mortem examinations for the police.
Two cancer deaths have been identified in the investigations and these "complex cases" are being further scrutinised.
As a result of the errors, Dr Kiberu has been suspended from his post as a research assistant at the Pilgrim hospital, Boston, Lincolnshire, and reported to the General Medical Council.
Last night, the Secretary of State for Health, Stephen Dorrell, said in a statement: "I have great sympathy with the patients involved, and their relatives. I have asked for reports on Dr Kiberu's work in the NHS and I have no further comment to make on this case until I have studied those reports."
In three hospitals - Grantham, Bassetlaw, and Victoria Kirkaldy - more than 4,500 laboratory specimens have already been checked and 602 errors have been detected. In 89 cases the errors were found to be serious. In another three hospitals - Luton and Dunstable, Central Middlesex, and Grimsby - hundreds more specimens are being checked.
Professor Alastair Bellingham, president of the Royal College of Pathologists, said yesterday they had long argued for tighter procedures to control the appointment of locums. "When the college initially heard of possible errors in the work of Dr Kiberu it, on request, recommended experts to review the cases on which the locum had worked."
Dr Kiberu, who graduated in 1975 in Dar-es-Salam, Tanzania, became a member of the royal college in 1990 following five years' postgraduate training and examinations.
Keith Lockyer, secretary of the college, said that while consultant posts are filled after exacting, statutory appointments procedures, locums do not have to undergo such rigorous assessment. "The 'MRCPath' [member of the Royal College of Pathologists] says that you have achieved the required standards of competence as a pathologist, but it does not say you are expert in every specialist field of pathology. Locums can be appointed without hospitals checking that they have the right expertise."
Dr Kiberu worked at Grantham District hospital for 10 months in 1991- 92 as a consultant locum in charge of the laboratory. Some 2,750 tissue samples have been reassessed at a cost of pounds 22,000; 474 errors were detected in 428 patients and 74 patients have been asked to make new appointments.
The pathologist worked at Bassetlaw hospital for six months to March 1993. There, 1,509 specimens were reviewed and 127 errors detected. At Victoria hospital, Kirkaldy, where the pathologist worked for two weeks in July 1991, no errors were found.
n Hospital helplines: Grantham - 0500 888488; Bassetlaw - 0800 502650; Luton and Dunstable - 0500 222001; and Grimsby - 01472 875348.Reuse content