The disgraced former Telegraph owner Conrad Black and the Enron fraudster Jeffrey Skilling have won fresh hope of early release from jail following a US supreme court ruling that their convictions partly relied on a controversial corruption law that was too broad in its scope.

In a major legal victory for the two jailed tycoons, America's top court issued separate, but related, rulings declaring that the men were treated unfairly when appeal court judges threw out their attempts to overturn their convictions.

However, the rulings shed doubt only on certain aspects of the men's multiple convictions and stop well short of acquittal.

Black, currently an inmate at Florida's Coleman prison, was sentenced in 2007 to six and a half years for defrauding shareholders in his Hollinger media empire out of $6.1m (£3.7m) by attaching a "non-compete" clauses to the sale of newspaper businesses that siphoned off funds from investors. The Canadian-born peer was stripped of the Conservative whip following his conviction. He has vigorously protested his innocence from the beginning.

Skilling, 56, is in a prison near Denver and is serving a 24-year sentence. He was chief executive of Enron until shortly before the energy trading company imploded in one of the most dramatic corporate corruption scandals in US history.

In both cases, prosecutors used a law that allows for conviction if business leaders are found to have robbed investors of "honest services". But twin decisions written by the supreme court judge Ruth Bader Ginsburg rule that this law should only be applied to incidents of bribery and kickback schemes.

Referring to Black, the ruling concludes: "We vacate the judgment of the court of appeals and remand the case for further proceedings."

The decision affects three counts of fraud for which Black was convicted by a Chicago jury. But the 65-year-old former press baron still faces a hurdle in that he was also found guilty of obstructing justice by removing boxes of evidence from his Toronto office in defiance of a court order. Once friendly with society figures ranging from Margaret Thatcher to Henry Kissinger, Black described the case against him as hanging "like a toilet seat" around the necks of prosecutors.

His legal team have never been pleased to have their case lumped in with Skilling, even through they appealed almost simultaneously to the supreme court on the same legal grounds. Skilling was convicted in 2006 on 19 counts of fraud, conspiracy, insider trading and lying to auditors. His co-accused, the former Enron chairman Ken Lay, died of a heart attack while awaiting sentencing.

Ginsburg's ruling, which is written on behalf of a majority of the supreme court's nine judges, says her decision does not necessarily mean the Enron boss's convictions should be overturned. And she threw out a claim by Skilling that his trial was unfair because it was held in Houston, a city scarred by the energy trading company's demise.

• This article was amended on Friday June 25 2010. The US supreme court has nine judges, not 11.