Serious assaults on prison officers have increased by nearly half since the coalition took power, the government admitted yesterday.

A total of 543 assaults by prisoners on officers were referred to the police in 2012 - three attacks every two days and 45 per cent more than the 374 in 2010.

But prison officers union POA said the real figures were "significantly higher" because assaults are underreported.

"We question the validity of the figures due to underreporting and the systems used to report assaults," a POA spokesman said.

Part of the cause was the government's relentless privatisation of Britain's prisons, the union said.

"The POA does not blame private companies but holds the government accountable for introducing the privatisation agenda.

"That agenda has failed and caused issues to be consistently shrouded by a cloak of corporate confidentiality."

Shadow justice minister Sadiq Khan pointed to a "crisis" of overcrowding in prisons caused by the coalition.

"How can ministers expect to rehabilitate criminals if prisons are dens of violence?" he demanded.

"It's not an overstatement to say that prisons are in crisis and the government is either oblivious or simply doesn't care."

Prisons Minister Jeremy Wright said the National Offender Management Service is reviewing policy for managing violence in prisons.