Boris Johnson has entered the debate over what should happen to the Royal Marine who murdered a wounded Afghan insurgent.
Marine A was convicted of killing the man after footage emerged of him shooting the man at close range in the centre of his chest.
It has prompted a furious debate about whether Marine A should be shown leniency, given the conditions under which he was operating.
Maj Gen Thompson, who led 3 Commando Brigade during the Falklands War, refused to condemn his actions and told The Times that a five-year prison term would be more suitable than life imprisonment.
But General Sir Nicholas Houghton, the Chief of the Defence Staff of the British Armed Forces, told the Marr show: "This was a heinous crime. Judicial process has found this individual guilty. It would be quite wrong for the armed forces to adopt some special pleading, some sort of exemption."
In his Daily Telegraph column on Monday morning, Johnson said the law should be upheld, but suggested that applying human rights legislation to the battlefield could cause problems for Britain's military.
Claiming the UK was "out on a limb" in this field, Johnson wrote that the US "whacked bin Laden without a second thought; they execute whole families in drone strikes."
He went on: "The risk for Britain is clearly that it will be harder and harder for the Armed Forces to conduct operations without legal paranoia, paralysis and expense."