A second prisoner has died in the wake of a weekend riot at a federal prison in B.C.'s Fraser Valley.

The unidentified prisoner died of a drug overdose on Sunday after being taken to hospital, prison officials said Tuesday morning.

Union officials have told CBC News that more than 10 other inmates were rushed to hospital after overdosing on drugs stolen from the prison hospital during the riot Saturday night.

"There was several inmates that actually broke into the hospital unit," said Gord Robertson, the regional president with the Union of Canadian Correctional Officers.

"They managed to gain access to a variety of drugs — methadone, to just over-the-counter prescription-type drugs would be housed in there."

On Sunday, prison officials said convicted child sex-offender Michael Gibbon was beaten to death during the riot, which was the third disturbance at the Mountain Institution this year.

Mountain Institution is a medium-security facility 120 kilometres east of Vancouver that houses almost 500 federally sentenced male inmates.

Gibbon's killing and the overdoses show the need for tough new restrictions, said Robertson.

The union is calling for the construction of barriers so inmates can be segregated to prevent another full-scale riot.

Prison guards said Corrections Canada was warned about potential problems with the prison's open concept, where inmates can move freely within the prison walls.

On Saturday night, guards fled to the prison roof after inmates began rebelling in the gymnasium, later taking over the entire prison before extra guards were sent in to take control.

With files from the Canadian Press