Frontline prison officer numbers fell by more than 300 last year, figures have revealed, sparking fresh scrutiny of staffing levels as jails tackle a safety crisis.
At the end of December there were 17,888 personnel in key operational roles in public sector jails in England and Wales.
This was a fall of 347 (1.9%) year-on-year, and 115 (0.6%) down on the tally in September, statistics published by the Ministry of Justice show.
The numbers relate to staffing bands 3 to 5, which consist of officers, specialists, supervising officers and custodial managers.
On a like-for-like basis, there has been a reduction of 6,450 "full-time equivalent" officers in the bands between March 2010 and December.
The figures showed a shortfall of 983 staff in the frontline categories against target levels at the end of last year.
The leaving rate for band 3 to 5 officers was 9% - almost double the level in 2012/13 (4.6%).
Leaving rates are calculated as the percentage of staff with a permanent contract of employment who left for reasons other than voluntary early departure schemes and redundancy.
Shrinking staffing levels have come under the spotlight in recent months as assaults and suicides behind bars reached record levels.
Shadow prisons minister Yasmin Qureshi said: "We need urgent action to tackle the under-staffing which is contributing to the Tories' prisons crisis."
Ministers have announced a recruitment drive as part of prison reform measures.
An MoJ spokeswoman said: "The Justice Secretary has been clear that we need to recruit more prison officers if we are to transform prisons into places of safety and reform.
"We've taken immediate action to boost staff at 10 of the most challenging prisons with 389 job offers now made -- meaning we are well on track to recruit the first 400 of the extra 2,500 frontline officers committed by the end of next month."
The spokeswoman said governors are being given more freedom and flexibility over local recruitment, while new schemes have been launched to attract top graduates and former military personnel into the service.