As deputy property manager within the Royal Household, Ronald Harper had an annual budget of £2.3m and was able to authorise orders worth up to £30,000.
During his employment, Harper personally received more than £100,000 in kickbacks from companies that were awarded lucrative and prestigious M&E contracts in Buckingham Palace, the Queen's Gallery, St James's Palace and Kensington Palace.
One company renewed its Royal Warrant during the period of the conspiracy.
In sentencing, the judge commented that Harper's offences were aggravated by the fact that he proudly displayed an award for excellence, given by the Royal Household, in his office.
Christopher Murphy and Aseai Zlaoui were found guilty of conspiracy to make corrupt payments. Steven Thompson and Glynn Orridge pleaded guilty to conspiracy to commit fraud.
Crown Prosecution Service head of special crime Nick Vamos said: "This was a complex scam but by working in close liaison with Leicestershire Police, CPS prosecutors were able to meticulously piece together a case that laid bare the extent of the defendants' corruption.
"It was a long-running, sophisticated and well-planned fraud in which they exploited the good name and status of the Royal Household to enrich themselves at the taxpayers' expense.”
Harper was found guilty of conspiring to receive corrupt payments from the former owners of Melton Power Services (MPS) and BSI Nordale. During two trials at Southwark Crown Court, the court heard that the payments were made to maintain good relations with Harper and to obtain or retain the valuable contracts the companies had with the Royal Household.
Evidence put before the court showed that the tender processes and payment systems were manipulated, invoices were fabricated and that a Royal Warrant was granted to MPS on the recommendation of Ronald Harper. As a consequence of the conspiracies the Royal Household was overcharged for the necessary work which was undertaken at the Royal Palaces.
Both MPS and BSI inflated the price of contracts and the additional amounts were then used to create a fund from which the bribes were paid. Thompson, the former owner of MPS, also kept some of the additional money himself.
Thompson pleaded guilty to his role in the making of corrupt payments to Harper. He and Orridge – a subcontractor to MPS – also pleaded guilty to defrauding MPS.
Murphy, who had worked with Harper in a similar role at Harrods department store, and Zlaoui were found guilty of conspiring with Harper to make corrupt payments at the time their company BSI Nordale was awarded large contracts to work at Buckingham Palace and St James's Palace.
Some of the payments by BSI Nordale to Harper were disguised using his brother-in-law Alan Rollinson, who was convicted of money laundering.
Harper received £55,000 in covert payments from MPS and £20,000 from BSI Nordale. In addition unexplained cash deposits into Harper's bank account in the region of £30,000 were also uncovered.