Last week Mr Dierich revealed that he and Jospin had been members of the same OCI cell in 1971, in which Jospin went under the code-name "Comrade Michel."
"It is true that in the 1960s I took an interest in Trotskyist ideas, and I established relations with one of the groups of this political movement," he said.
"It was a personal, intellectual and political journey of which I am not in the least ashamed."
Anti-Stalinist
Explaining his allegiance, he said that "in the very different world that was the 1960s, two elements were essential in my political development - anti-colonialism and anti-Stalinism."
Mr Jospin had previously denied he had been a member of a Trotskyist party, saying the recurring allegations were founded on a confusion with his brother Olivier.
Comrade Michel
But a former member of the OCI, Patrick Dierich, as well as several interviewees quoted anonymously by Le Monde, all said there was no doubt about his identity.
Last week Mr Dierich revealed that he and Jospin had been members of the same OCI cell in 1971, in which Jospin went under the code-name "Comrade Michel."
One of the founders of OCI, Boris Fraenkel, told the French magazine Le Nouvel Observateur that he had inducted Jospin into the party in 1964.
"Lionel Jospin came regularly to see me... to take part in a revolutionary studies group. This was the necessary training before joining the Trotskyist movement," Mr Fraenkel said in an interview posted on the magazine's website.
Mr Fraenkel said he trained Mr Jospin while the latter was a student at the elite civil service training school, Ecole Nationale D'Administration (ENA).
"I trained him up in secret. A future top civil servant wouldn't want to be known as a revolutionary! At that point we had no ENA graduate in the movement. It was an extraordinary opportunity to penetrate the country's administrative class."
Germany's Fischer
About six months ago, German Foreign Minister Joschka Fischer found himself in a similar position to Mr Jospin's, when he had to defend his past as a radical activist in parliament.
Conservatives had demanded his resignation, but Mr Fischer remained in office and remained popular.
The truth about Mr Jospin's early political career was becoming increasingly sensitive with the approach of presidential elections next year, in which he is expected to stand as the Socialist Party candidate.
According to Le Monde, the OCI had a policy of secretly infiltrating French institutions and throughout the 1970s its leader, Pierre Lambert, kept in close contact with Mr Jospin at the Socialist Party. The final break only came in 1987.
Right-wing deputy Renaud Donnedieu de Vabres urged Jospin to "take responsibility for his past," and admit he was a member of the OCI. "What's unacceptable is the lying," he said.