The Prime Minister says Iran's admission it shot down a Ukrainian passenger plane is an "important first step".
Boris Johnson said the statement from Iran "reinforces the importance of de-escalating tensions in the region".
The aircraft came down after taking off from Tehran on Wednesday, against the backdrop of heightened US-Iran tensions.
One hundred and seventy six people lost their lives in the disaster.
"This will be an incredibly difficult time for all those families who lost loved ones in such tragic circumstances," Mr Johnson said.
"We will do everything we can to support the families of the British victims and ensure they get the answers and closure they deserve."
The Prime Minister is calling for a "comprehensive, transparent and independent international investigation and the repatriation of those who died."
He went on to say, "the UK will work closely with Canada, Ukraine and our other international partners affected by this accident to ensure this happens."
Addressing a "no war with Iran" protest in Trafalgar Square on Saturday, Labour leader Jeremy Corbyn said the shooting down of the jet was an "appalling act" for which there can be "no excuses".
The Queen has said her thoughts and prayers are with the people of Canada, saying "Philip and I have been deeply saddened by the tragic loss of life".
Cover image: Prime Minister Boris Johnson outside 10 Downing Street (Picture: PA).