Pc Dustin Hunter, 30, formerly of Surrey Police, provided stolen police uniforms as well as detailed plans on how to avoid getting caught.
But he was unaware the man he was assisting to carry out the supposed raid was actually an undercover policeman.
Last month, the 30-year-old admitted encouraging or assisting in a robbery.
Police launched the undercover investigation into his activities after he was arrested for possession with intent to supply 994g of cannabis with a street value of 5,670 which was found in his car in November last year.
Sentencing at Harrow Crown Court on both matters yesterday, the judge, Recorder Michael Caplan QC, said: "You have shown complete contempt for law and order, which you as a police officer had pledged to uphold."
Hunter, formerly of Budgen Close, Crawley, West Sussex, was sentenced to two years for the drugs offence and five years for his part in planning the robbery.
The court heard Hunter was contacted by a man, known only as John, in January this year.
Hunter, who had worked in the police intelligence unit, suggested a number of possible crimes they could commit, including robbing known drug dealers, dealing controlled drugs, setting up a cannabis factory and stealing from online betting accounts.
He also told "John" he could provide information on a foiled robbery attempt at Decorus jewellers, and that he had access to police uniforms.
During several further meetings at the same pub, the pair planned a robbery at the same jewellers.
Michael Boardman, defending, said Hunter had made sure no firearms were going to be used in the robbery and said the night before it was set to take place, he tried to stop the raid going ahead.