The Court of Appeal upheld complaints by Eileen Dallaglio and Margaret Lockwood-Croft against Dr Paul Knapman, the Westminster coroner who conducted the basic inquest into the tragedy in which 51 people were killed when the pleasure boat collided with a dredger, the Bowbelle, on the Thames in August 1989.
Lord Justice Simon Brown said Dr Knapman showed 'apparent bias' in describing Mrs Lockwood-Croft, who lost her son Shaun, 26, in the accident, as 'unhinged'.
The description, in a newspaper article, was not merely injudicious and insensitive but bound to be interpreted as a gratuitous insult, he added.
Mrs Lockwood-Croft, of Aldershot, Hampshire, said she was 'thrilled to bits' to win such a ruling after fighting for nearly four years.
'I hope we can now have the case considered by a caring and compassionate coroner who will make recommendations on health and safety on the river Thames,' she said.
Mrs Dallaglio, of Barnes, south-west London, who lost her 19-year-old daughter Francesca, said after the ruling: 'This has restored my faith in British justice. I am overwhelmed.'
The court said there was a real possibility that Dr Knapman, in refusing to reopen the inquests was unconsciously influenced against members of the Marchioness Action Group by a feeling of hostility towards them.
It was possible the coroner thought them troublemakers and unfairly undervalued their argument for resumed inquests said the judge, sitting with Master of the Rolls Sir Thomas Bingham and Lord Justice Farquharson.
The case was referred to a different coroner in a different district for a decision on whether the inquests should be resumed and, if so, in respect of how many of the dead. Thirty-six families are believed to be in favour of a further hearing.
But the court warned the families against undue optimism that another coroner would take a different view on resumption of the inquests. There were powerful grounds against resuming the hearing, including the interests of those families who had put the tragedy behind them.
A request to re-open the inquest was turned down by Dr Knapman in 1992 after juries failed to reach verdicts in two sets of criminal proceedings. The original inquest was adjourned in 1990 because of the criminal cases. A public inquiry has never been held.
Lawyers for Dr Knapman are to consider applying to the House of Lords for leave to appeal against yesterday's ruling.Reuse content