The UK informed the United Nations about its drone strike on an Islamic State militant in Syria, claiming the hit had been a "necessary and proportionate" action.
In the letter, dated yesterday - the same day Prime Minister David Cameron revealed to MPs that the strike had taken place - representative Matthew Rycroft said the target was "known to be actively engaged in planning and directing imminent armed attacks" against the UK.
The targeted attack in Syria last month killed two fighters for the so-called Islamic State, Reeyad Khan and Ruhul Amin, who were both from the UK.
Mr Rycroft said it had been in the interests of self-defence for Britain, as well as the collective defence of Iraq, where the extremist group has taken over swathes of land.
Defence Secretary Michael Fallon has suggested there were more names on the list who were legitimate targets for air strikes.
The daughter of murdered aid worker David Haines tells ITV News drone strikes against Islamic State are "a step in the right direction".
Two British Islamic State fighters were killed by a RAF drone strike in Syria, while a third died during an American attack.