She said she had been deeply moved by how the infected blood had devastated victims’ lives. She invoked Glenn Wilkinson, a father of two, who has hemophilia and was infected with hepatitis C when he was 19 after a routine operation in 1983 to remove two teeth.

Mr. Wilkinson, one of dozens of people who have been campaigning for an inquiry and compensation, had described how the specter of illness had dominated his life and how for years he lived in fear of passing the virus on to family members.

Hemophilia, an inherited disorder that prevents blood from clotting properly, was initially controlled using blood plasma products that encouraged clotting. Later, synthetic clotting factor proteins were developed. But in the 1970s and ’80s, screening procedures were insufficient. Ms. Johnson noted that blood contamination scandals had been investigated in France and Japan, among other countries, and led to some convictions of government health officials. But in Britain, she said, officials had not been held to account.

The inquiry comes after leaders of all the main political parties wrote a letter this month to Mrs. May demanding a full investigation amid longstanding concerns. Victims’ advocates have contended for years that, among other things, the decisions that led to the deaths had been covered up, that the Department of Health had evaded responsibility, and that important evidence, including the medical records of those affected, had been tampered with.

The letter alluded to accusations that Department of Health officials destroyed documents to hide what had happened, and also said that contaminated blood was “not removed from the blood supply, once the dangers became known.”