The case against a former IRA commander accused of involvement in the murder of a Belfast widow who was kidnapped, killed and buried in secret has collapsed.

Ivor Bell, a veteran republican, was cleared of soliciting the murder of Jean McConville in 1972. McConville became known as one of the “disappeared” of the Northern Ireland Troubles.

The prosecution’s case against 82-year-old Bell, from west Belfast, was based on alleged admissions he gave to the Boston College/Belfast Project tapes, an archive of taped testimonies from IRA and loyalist paramilitaries who admitted their part in terrorist crimes including murder during the Troubles.

Detectives from the Police Service of Northern Ireland went to the US supreme court to seize the Boston tapes and used them to prosecute Bell, claiming he was “interviewee Z”. This interview contained accounts of how the IRA killed McConville and hid her body.

But a judge in Belfast crown court on Thursday said Bell could not be found guilty. Mr Justice O’Hara told the jury: “As a result of some legal rulings which have been made over the last two days there is now no evidence that the prosecution can put before you to support the case it was putting against Mr Bell. My ruling now is to direct you to return a verdict of not guilty because you simply cannot find him to have done the acts alleged.”

The judge lifted reporting restrictions that had prohibited the reporting of the case.

The restrictions had prevented the Guardian from revealing earlier this year that a key American academic could no longer testify in the Bell trial and other upcoming cases connected to the Boston College tapes.

The Guardian obtained legal documents showing that the head of the US university library where the tapes of self-confessed IRA and UVF activists were held had won the right to withdraw from criminal cases relating to the oral history programme.

Legal sources and campaigners on both sides of the Atlantic claimed the withdrawal on health grounds of Bob O’Neill would potentially collapse some of these cases. O’Neill is understood to have a deteriorating heath condition and his Boston-based lawyers argued he was unfit to take part in any trials.

O’Neill’s evidence was regarded as crucial given that he was the person who allegedly knew the identities of all the former paramilitaries on the tapes. None of the participants gave their names on the recordings.

Bell’s solicitor, Peter Corrigan, of Phoenix Law, said: “The Boston tapes were of no benefit from a historical perspective, never mind meeting the threshold of evidence in a criminal trial. The process from start to finish was fatally flawed, which lacked the relevant safeguards and as described by one expert during the course of the trial, ‘is exactly not how to conduct an oral history project’.”