Jeremy Corbyn remained silent while the national anthem was sung at an RAF service marking the 75th anniversary of the Battle of Britain.
In photographs taken during the St Paul's Cathedral event, the Labour leader was seen standing solemnly near David Cameron, as those around him sang God Save The Queen.
In a statement released before the event, Mr Corbyn paid tribute to the RAF and its role in the Battle of Britain.
“My mum served as an air raid warden and my dad in the Home Guard," he said.
“Like that whole generation, they showed tremendous courage and determination to defeat fascism. The heroism of the Royal Air Force in the Battle of Britain is something to which we all owe an enormous debt of gratitude.
“The loss of life - both civilian and military - should be commemorated so that we both honour their lives and do all that we can to ensure future generations are spared the horrors of war.”
In another photo of the ceremony, Mr Corbyn does appear to be singing.
A republican, Mr Corbyn has in the past said he would rather the head of state was not a hereditary role.
Since winning a landslide victory in the Labour leadership election at the weekend, the leader of the opposition has accepted an invitation to become a member of the Queen's privy council.
The service was Mr Corbyn's first official occasion as Labour leader.
A spokesperson for Jeremy Corbyn said: “Jeremy attended today’s event to show respect for those who fought in conflicts for Britain.
“As he said in the words issued this morning, the heroism of the Royal Air Force in the Battle of Britain is something to which we all owe an enormous debt of gratitude.
“He stood in respectful silence during the anthem.”
Additional reporting by PA
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