In the first recorded case of its kind, staff on patrol discovered the three rodents just inside the perimeter fence at HMP Guys Marsh in Dorset.
Noticing that the rats appeared to have been stitched along their stomachs, the officers opened them up.
They found five mobile phones and chargers, three SIM cards, cigarette papers, and drugs including Spice and cannabis.
Intelligence suggests the rats were thrown over the prison fence by organised criminals, who co-ordinated with an offender on the inside who was waiting to collect them, according to the Ministry of Justice.
Had they not been seized, the items would have been sold around the jail “leaving chaos and violence in their wake”, officials said.
The prison service is working with police to find and prosecute the culprits.
Prisons Minister Rory Stewart said: “This find shows the extraordinary lengths to which criminals will go to smuggle drugs into prison, and underlines why our work to improve security is so important.
“Drugs and mobile phones behind bars put prisoners, prison officers and the public at risk.
“By toughening security and searching, we can ensure prisons are places of rehabilitation that will prevent further re-offending and keep the public safe.”
While tennis balls and pigeons have been used to get contraband into prisons, the MoJ said the find at HMP Guys Marsh earlier this month was the first recorded instance involving rats.
Drugs such as Spice have been identified as a key factor behind the safety crisis that has swept through the jail estate.
Establishments are also attempting to stem the flow of illegal phones amid concerns they are used to facilitate more crime and intimidate victims from behind bars.
In the 12 months to March 2018, there were 13,119 incidents where drugs were found in prisons in England and Wales - a rise of nearly a quarter (23%) on the previous year.
Discoveries of mobile phones also increased, going up by 15% to 10,643 instances in 2017-18.
Once banned items make it in, officials say inmates are adopting increasingly inventive tactics to keep them hidden, such as by secreting them in hollowed out furniture or in electrical items.
A host of measures are being deployed as part of a multimillion pound drive to boost prison security, including scanning equipment to detect drugs on people or in mail, phone-blocking technology and improved searching techniques.
HMP Guys Marsh, a category C training prison for adult male inmates, had a population of 384 as of the end of last month.
It has been the subject of a number of critical watchdog assessments.
Last year the prison’s Independent Monitoring Board reported that prisoners have “easy access” to mobile phones and “make calls at times to suit themselves”.
In 2017, a report from HM Inspectorate of Prisons detailed how inmates were able walk around the jail in dressing gowns or just shorts.
Sign up to our newsletter: enter your email in the box at the top of this article to get daily updates straight to your inbox.