An "unnecessary and unintentional" delay meant government intelligence of terrorists planning to kidnap British workers in Algeria was not passed on before a siege which left 40 people dead, an inquest has heard.
Scottish contractors Carson Bilsland, from Perthshire, and Kenny Whiteside, from Fife, were among seven Britons killed in the stand-off at the In Amenas plant in January 2013.
Flaws in the oil plant's security and risk assessment were also highlighted during the conclusion of the 30-day inquiry into the British deaths in London.
Judge John Hilliard found that Mr Bilsland, 46, was unlawfully killed by an explosion during the four-day siege by al Qaeda gunmen in 2013, while 59-year-old Mr Whiteside was unlawfully killed "while being held as a human shield".
A further eight Scots escaped from the remote In Amenas facility in eastern Algeria.
The crisis began on January 16 2013 when militants attacked two buses carrying workers to the plant. They then broke into the facility's living quarters and main installation, taking dozens of staff hostage.
In total, 40 employees and 29 militants died before the Algerian army regained control of the plant on January 19.
The coroner revealed that British Government intelligence received a tip in December 2012 alleging that unidentified terrorists were plotting to kidnap British workers in Algeria.
The Government later assessed the information as "questionable", but Judge Hilliard criticised "an unnecessary and unintentional delay in processing the information until after the attack had started".
The inquest at the Old Bailey also heard that the terrorists went "unhindered and unnoticed" across the Sahara to the facility, entering the compound via a gate that was left open every morning between 5.45 and 6.10am.
Judge Hilliard said the the attack had been aided by inside knowledge and documents.
The coroner also questioned why security had not been beefed up at the facility amid a growing threat to foreign nationals from Islamic terrorism.
Standard precautions such as reinforced steel gates and speed bumps to slow approaching vehicles were absent, and there were no armed guards on the gates of the living quarters or in the 12 watchtowers circling the compound, which was jointly operated by BP, the Algerian state company Sonatrach and the Norwegian company Statoil.
Internal security was handled by a civilian Algerian team, but the inquest highlighted a number of safety breaches.
Judge Hilliard said: "The evidence was that searches were not always carried out thoroughly and that guards would allow some Algerian workers through without searching them at all."
The terrorists, armed with heavy missiles and AK-47s, entered the complex undetected and began targetting French, British and American expatriates for hostages.
The inquest heard that inspection engineer Carson Bilsland, originally from Bridge of Cally near Blairgowrie, was captured because he saw Algerian workers being allowed to leave and thought it was safe. He was taken hostage and died inside a vehicle on January 17, the inquest was told.
Kenny Whiteside, described as "friendly and easygoing", was shot dead the same day. Originally from Glenrothes, he had lived in South Africa for 30 years with his wife and two daughters.
The other British victims were Sebastian John, from Norfolk; Stephen Green, from Hampshire; Paul Morgan and Garry Barlow, both from Liverpool; and Carlos Estrada Valencia, a Colombian national living in London.
BP said it would "closely review" the findings.
In a statement, it said: "Since the attack, BP and Statoil jointly carried out an extensive review of security arrangements in Algeria and have been working with the Algerian state energy company Sonatrach on a programme of security enhancements.
"The focus of BP and our partners in the In Amenas joint venture is to do all we can to prevent any such incident from happening again.
"Our thoughts remain with the families and friends of everyone who was affected by this tragedy - we will always remember them at BP."
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