LONDON -- A British judge sentenced a notorious serial rapist to at least 27 years in prison Friday for a spate of attacks on elderly people that lasted almost two decades.

Delroy Grant is seen in this undated photograph received from the Metropolitan Police in London March 24, 2011.

Delroy Grant,dubbed the "Night Stalker", was convicted of attacks on 18 elderly men and women between 1992 and 2009 following a lengthy investigation. Police say the number of Grant's victims is thought to be substantially more than that.

Judge Peter Rook said he considered giving Grant a life sentence _ meaning he could never be released _ and noted the crimes were so serious they were not covered by normal sentencing guidelines.

"Your offending is in a league of its own," he said at Woolwich Crown Court after hearing statements from Grant's victims and their families.

The attacker broke into homes, cut phone lines and crept into the bedrooms of his elderly victims during ordeals that lasted for hours. The victims were mostly elderly women; many were blind, deaf or had diseases such as Alzheimer's or Parkinson's. Some said the masked man woke them up by shining a flashlight in their faces.

The judge said Grant remained "a very dangerous man capable of committing heinous crimes" and that the 53-year-old will not be eligible for parole for 27 years.

Police have described Grant's offenses as some of the most "awful and disturbing crimes" Scotland Yard has ever investigated.

Grant, a former cab driver, was arrested in 2009 following one of Britain's longest-running manhunts. Police have apologized for missing earlier opportunities to catch Grant, saying the investigation was undermined by "basic policing errors."

The Independent Police Complaints Commission said Friday a poor response to a burglary and confusion over a suspect _ which saw the wrong man's DNA studied _ prevented Grant from being arrested sooner.

"It is clear that a simple misunderstanding had horrific consequences," said Deborah Glass, the watchdog's commissioner for London.