Aflytninger fra GCHQ af interne kommunikationer af kongerigets General Intelligence Directorate afslørede ordrer fra et "medlem af den royale kreds" til at kidnappe den problematiske journalist og at tage ham tilbage til Saudi Arabien.
Ordrerne, sagde efterretningskilderne, udgik ikke direkte fra den de facto hersker Crown Prince Mohammed bin Salman, og det er ikke bekendt om han var klar over disse.
Selv om de befalede at Khashoggi skulle kidnappes og føres tilbage til Riyadh, så "lod de døren stå åben" for andre handlinger hvis journalisten skulle blive besværlig, sagde kilderne.
Kommentar: Er det den britiske fortløbende indblanding, efter begivenhederne, for at beskytte MBS? Eller, hvis de kendte til hvad der var under opsejling, var briterne så medskyldige i mordet? Hvorfor advarede briterne ikke Khashoggi om hvad der var på færde eller i det mindste fraråde ham at besøge konsulatet?
I sidste uge bekræftede den saudi arabiske justitsminister at mordet var overlagt - i modsætning til den første officielle forklaring som var at Khashoggi var blevet dræbt efter at en slåskamp brød ud.
Kommentar: Denne artikel er delvis oversat til dansk af Sott.net fra: British intelligence community says it knew about Saudi plan to get Khashoggi THREE WEEKS beforehand
"The suspects in the incident had committed their act with a premeditated intention," he said.
"The Public Prosecution continues its investigations with the accused in the light of what it has received and the results of its investigations to reach facts and complete the course of justice."
Those suspects are within a 15-strong hit squad sent to Turkey, and include serving members of GID.
Speaking last night the intelligence source told the Sunday Express: "We were initially made aware that something was going in the first week of September, around three weeks before Mr Khashoggi walked into the consulate on October 2, though it took more time for other details to emerge.
"These details included primary orders to capture Mr Khashoggi and bring him back to Saudi Arabia for questioning. However, the door seemed to be left open for alternative remedies to what was seen as a big problem.
"We know the orders came from a member of the royal circle but have no direct information to link them to Crown Prince Mohammad bin Salman.
"Whether this meant he was not the original issuer, we cannot say."
Crucially, the highly-placed source confirms that MI6 had warned its Saudi Arabian counterparts to cancel the mission - though this request was ignored.
"On October 1 we became aware of the movement of a group, which included members of Ri'āsat Al-Istikhbārāt Al-'Āmah (GID) to Istanbul, and it was pretty clear what their aim was.
"Through channels we warned that this was not a good idea. Subsequent events show that our warning was ignored."
Asked why MI6 had not alerted its Five Eye intelligence partner, the US (Khashoggi was a US citizen), the source said only: "A decision was taken that we'd done what we could."
Comment: He wasn't quite a US citizen, though his children are.
However analysts offered one possible explanation for this.
"The misleading image that has been created of Jamal Khashoggi covers up more than it reveals. As an insider to the Saudi regime, Khashoggi had also been close to the former head of the intelligence agency. He was an Islamist, a member of the Muslim Brotherhood, and someone who befriended Osama Bin Laden and had been sympathetic to his Jihad in Afghanistan," said Tom Wilson, of the Henry Jackson Society think-tank.
"All of these connections are being hidden by a simplistic narrative that Jamal Khashoggi was just a progressive freedom fighting journalist. It isn't plausible that he was murdered simply for being a journalist critical of the regime. The truth is much more complicated."
Last night a close friend of Mr Khashoggi revealed that he was about to obtain "documentary evidence" proving clams that Saudi Arabia had used chemical weapons in its proxy war in Yemen.
"I met him a week before his death. He was unhappy and he was worried," said the middle eastern academic, who did not wish to be named.
"When I asked him why he was worried, he didn't really want to reply, but eventually he told me he was getting proof that Saudi Arabia had used chemical weapons. He said he hoped he would be getting documentary evidence.
"All I can tell you is that the next thing I heard, he was missing."
While there have been recent unsubstantiated claims in Iran that Saudi Arabia has been supplying ingredients that can be used to produce the nerve agent Sarin in Yemen, it is more likely that Mr Khashoggi was referring to phosphorous.
Last month it was claimed that Saudi Arabia had been using US-supplied white phosphorous munitions against troops and even civilians in Yemen.
Though regulations state the chemical may be used to provide smokescreens, if used illegally it can it burn to the bone.
Chemical warfare expert Col Hamish de Bretton-Gordon said: "We have already seen in Syria that nothing is as effective as chemical weapons in clearing urban areas of troops and civilians - Assad has used phosphorous for this very reason.
"If Khashoggi did, in fact, have proof that Saudi Arabia was deliberately misusing phosphorous for this purpose, it would be highly embarrassing for the regime and provides the nearest motive yet as to why Riyadh may have acted when they did against him."
Kommentar: Maybe, in the sense of being 'the straw that broke the camel's back', but Saudi Arabia is already known to have provided ISIS and affiliated terrorists in Syria with chemical weapons. Also, the kingdom's 'big brothers' - the US and UK - use WP and other chemical agents in their wars as a matter of course.
Whatever their reason for sending 15 assassins with bone saws and black bags to butcher Khasoggi, the real bombshell here is that the British were so familiar with the Saudis' plan to 'abduct' Khashoggi that they warned them to not proceed because of the bad press they knew would follow...
Some psychopaths are evidently smarter than others.