Drones have increasingly been used to smuggle drugs, mobile phones and other contraband into jails, figures suggest.
Statistics obtained through a Press Association Freedom of Information request show that in 2013 none of the unmanned aircraft were detected in or around prisons in England and Wales.
This rose to two incidents in 2014 and 33 in 2015. Items discovered include drugs, phones and USB drives.
The government said incidents were rare.
Across the 35 incidents, drugs were discovered at least six times, mobile phones more than eight times and a drone itself recovered in 19 instances.
One of the biggest finds listed a drone, drugs, mobile phone, a charger and USB cards at HMP Oakwood in the West Midlands in December last year.
Five incidents were recorded as including "unknown packages", which the Ministry of Justice said referred to an item recovered as part of a suspected drone incident, with no specific information recorded on the contents.
A total of 14 incidents were recorded as "miscellaneous", which referred to a reported drone sighting in or around a prison.
The MoJ said that where an incident had been listed as this or as "drone only", it did not know if the craft was being used for illegal purposes.
Six incidents were listed as "drone only".
The MoJ added that while a drone was not always found, there may have been evidence of a drone having been used, such as on CCTV, or there may have been witnesses.
The location where the drones and packages were found varied from different places within prison grounds to just outside its walls.
HMP Onley in Northamptonshire had four drone incidents between 2014 and 2015, followed by Lindholme, Ranby and Swansea prisons on three, and Bedford, Wandsworth and Manchester on two.
Leicester, The Mount, Whatton, Leeds, Eastwood Park, Liverpool, Norwich, Glen Parva, Huntercombe, Wormwood Scrubs, Full Sutton, Guys Marsh, Long Lartin, Bullingdon, Wealstun and Oakwood prisons all recorded one incident.
Mike Rolfe, national chairman elect of the Prison Officers Association (POA), said the use of drones to smuggle in banned items was of "serious concern".
He called for more staff to tackle the issue, so they could carry out cell checks and prisoner searches to find parcels delivered by drone.
The MoJ said it took a "zero tolerance" approach to illicit material in prisons.
New legislation made it illegal to land a drone in prison or to use a drone to drop in psychoactive substances, it said.
And anyone found using drones in an attempt to get contraband into prisons could be punished with a sentence of up to two years.
A report published in December by HM Inspectorate of Prisons noted that illegal drugs, legal highs and illicit medications may get into prisons in a number of ways, meaning it is not always possible to quantify exactly how many drugs are making it into prisons.
The report stated that "easy access to illicit mobile telephones makes it possible to plan the drops carefully".