Authorities in British Columbia have had to drop one of the 27 murder charges laid against suspected serial killer Robert Pickton.

Pickton cannot be charged in the death of an unidentified woman, a judge ruled on Thursday.

Pickton, 56, has pleaded not guilty to the murder of the 26 named victims in the case.

Sketch of Robert Pickton (CP file photo)

He refused to enter a plea on the charge involving the unidentified woman, known in the proceedings as Jane Doe, so the court registered a not-guilty plea on his behalf.

That charge has now been thrown out, with Thursday's release of the judge's written ruling.

The count fails to meet the minimal requirements set out in the Criminal Code. "Accordingly, it must be quashed," wrote Justice James Williams.

The detailed reasons for the judge's ruling cannot be reported because of a publication ban covering this stage of the trial.

Pickton's trial began on Jan. 30, four years after his arrest by police investigating the disappearance of more than 60 Vancouver sex-trade workers.

Most of the victims were drug-addicted women who vanished over a period of years from the city's Downtown Eastside.