Anti-arms campaigners have condemned Charles and Camilla Windsor for sending an open message of support to the brutal Saudi Arabian regime.
The royals arrived in the kingdom today as part of a wider Middle East tour.
The visit comes almost exactly two years after the Saudi Arabian National Guard sent British-made armoured personnel vehicles into Bahrain to support the suppression of protests there.
Campaign Against Arms Trade (Caat) pointed out that one of the key listed themes of the visit was "military links between the Saudi and UK Armed Forces."
The group said that it suspected that the visit has been added to the prince's Middle East itinerary in an attempt to persuade the Saudi regime to finalise a contract for 48 Eurofighter Typhoon aircraft.
Prime Minister David Cameron failed to secure the deal during a trip in November. Caat suggested that the British government may believe that the Saudis will be more impressed with a royal.
Caat spokeswoman Kaye Stearman said: "The BAE Eurofighter deal is still under discussion, the Serious Fraud Office is investigating a second Saudi arms deal, and a parliamentary committee is undertaking a review of UK-Saudi relations.
"Added to this is the steady stream of news about human rights abuses and reports of unrest in Saudi Arabia. No wonder the Saudi rulers are feeling concerned - even insecure.
"The visit of Prince Charles is meant to reassure them that they still have the support of the UK government and that they should sign the Eurofighter Typhoon deal."
Over the past five years Britain has licensed almost £4 billion worth of weaponry to Saudi Arabia.