Police insist they have not had the wool pulled over their eyes by intelligence services over the death of codebreaker Gareth Williams.
Britain's enemies are "rubbing their hands with glee" over whistleblower Edward Snowden's NSA leaks, the MI6 chief said.
Secret files reveal drunken meeting between Stalin and Churchill, a cross-dressing spy and King Edward VIII's phone bug.
MI6 chief Sir John Sawers will stand down after five years in the post, the Foreign Office has confirmed.
Sir John is set to leave the Secret Intelligence Service in November, a spokesman said.
Prior to his appointment in 2009, he served as a career diplomat rising to become Britain's ambassador to the United Nations.
Previously Sir John spent three years as foreign policy adviser to the then-prime minister Tony Blair.
Spies are to be given a "licence to speed" for the first time, under changes to motoring laws.
Officers in MI5 and MI6 currently have to abide by the rules of the road even when national security is under threat.
But transport minister Robert Goodwill is to hand them the same exemptions as police, fire and ambulance drivers.
They will be able to break the speed limit once they have completed a training course in high-speed driving.
Vehicles used to carry organs for transplant, bomb disposal units, and mountain rescue teams are also set to be added to the list.
No request has been made to the Attorney General's office for him to apply to the High Court to have the inquest verdict into the death of Gareth Williams quashed.
Today, after a two-year investigation, the Metropolitan Police concluded it is "most probable" that no-one else was present when the MI6 codebreaker died. An earlier inquest concluded Mr Williams was "probably" killed unlawfully.
Deputy Assistant Commissioner Martin Hewitt, of the Metropolitan Police, says there remains "gaps in understanding" regarding the death of MI6 worker Gareth Williams.
But he has concluded, following two years of investigation, that it is "most probable" that the codebreaker was alone at the time of his death.
Deputy Assistant Commissioner Martin Hewitt said MI6 worker Gareth Williams' death was "most probably" an accident, although he admitted: "No evidence has been identified to establish the full circumstances of Gareth's death beyond all reasonable doubt."
Mr Williams, a fitness enthusiast originally from Anglesey, was found in the bag in the bath at his flat in Pimlico on August 23, 2010.
Pathologists said he would have suffocated within three minutes if he was alive when he got inside the 32-inch by 19-inch holdall.
None of his DNA was found on the lock on the bag and his palm prints were not found on the rim of the bath.
However, Mr Hewitt said there was no evidence that the flat had been "deep-cleaned" to remove forensic traces and nothing to suggest a struggle or a break-in.
He said it was "beyond credibility" that the secret services had "pulled the wool" over his eyes.
"I do not believe that I have had the wool pulled over my eyes. I believe that what we are dealing with is a tragic unexplained death."
The family of Gareth Williams have said they are "naturally disappointed" it is not possible to state with certainty how the codebreaker died.
They said in a statement they back the coroner's verdict that Gareth was probably unlawfully killed over the latest police conclusions.
They said: "We consider that on the basis of the facts at present known the Coroner's verdict accurately reflects the circumstances of Gareth's death."
They added: "We still however remain very disappointed over the failure of his employers at MI6 to take even the most basic enquiries concerning Gareth's welfare when he failed to attend for work on the 16th August, 2010.
"We believe that if proper steps had been taken in the same manner as any reasonable employer would have undertaken further information relating to the cause of his death might have become apparent and not have been lost due to the length of time before Gareth's body was found."
Spy's family disagree with new police conclusion. They back coroner's view prob unlawfully killed. Again criticise MI6 failures after death
Reconsidering all the original evidence and following new lines of inquiry, police say there is still insufficient evidence to be definitive on the circumstances of Gareth Williams' death.
They say it is theoretically possible for a person to lock themselves in a bag with the sort of lock used.
There is no evidence to support the theory there was a forensic clean of fingerprints
There is no evidence of a third person in the flat
No evidence of forced entry
Police say they have had full support from security services and there is nothing to support the theory that Mr Williams' death was related to his work.
His interest in womens' clothes fuelled much speculation, they say, but the clothing had no bearing on the circumstances of his death.
Officers say Mr Williams was a very private person, who was close to his family and had few close friends. He was concientious and was a decent man with hobbies.
Police explained no evidence has been identified that could establish the full circumstances of Mr Williams' death beyond reasonable doubt.
They say, on balance, their probable conclusion is that no person was present when he died and there is insuffient new evidence to re-open an inquest.
There are no more active lines of inquiry but police say the case will remain under review.
Working at MI6 is not like James Bond, one of the security chiefs told MPs that operatives are given far more supervision.
"The idea of sending an agent off into the field like James Bond, then he comes back two months later and reports... that doesn't work that way," Sir John Sawers, chief of MI6, said. "Our people in the field will have constant communication with us."