A prosecutor has urged a judge to ignore sentencing guidelines and throw the book at a gang of robbers who stole millions from a safe-deposit company.

Seven men, aged between 49 and 77, are being sentenced for the April heist in London's Hatton Garden diamond district.

The gang drilled through a concrete vault wall and ransacked more than 70 safe-deposit boxes containing 14 million pounds ($20 million) in cash, jewelry and gold.

Prosecutor Philip Evans said "this was a plan of the utmost sophistication" and urged sentences longer than the usual maximum of 10 years.

The oldest defendant, 77-year-old Brian Reader, was too ill to appear in court Monday. His lawyer said he had suffered a stroke in prison and might have only months to live.

The hearing continues Tuesday.