The skies over southern England have been filled with the sound of Merlin engines to mark 75 years since the Battle of Britain's 'Hardest Day'.

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On August 18, 1940 the German Luftwaffe flew hundreds of sorties across the English Channel in a bid to destroy the Royal Air Force's Fighter Command.

Both sides suffered heavily, losing about 70 aircraft each, but The Hardest Day is still seen as the moment Britain put paid to Hitler's ambitions of invading the UK.

To remember the day, more than 20 Spitfires and Hurricanes took off from the former RAF base at Biggin Hill in Kent.

Battle of Britain pilot Tony Pickering, 94, poses for a photograph in front a Hurricane fighter plane. ( Reuters: Peter Nicholls )

The average age of the pilots who flew in the Battle of Britain was just 20. Tony Pickering was one of them.

"When you're about 19 or 20 years of age, you've been trained to think about the job you've got to do, and that's all you think about," he said.

"I certainly remember it alright. And I remember the Germans picking us off before we even got to the bombers. It was quite exciting times, I can assure you."

Spitfires and a Hurricane fly in formation to mark the Battle of Britain's Hardest Day. ( Reuters: Peter Nicholls )

Flypast organiser Colin Hitchin said during the war pilots who took off from the airfield could not be sure they would be able to return.

"If you look over my shoulder you can see London. Those forces that came to destroy the Royal Air Force in 1940 crossed the Channel, and the next stop was Biggin Hill," he said.

"It really was our own ring of steel. And I think the Royal Air Force hung on by the skin of its teeth and did us great, you know, very proud.

"When those pilots took off from here, they didn't even know if they was going to come back to an airfield they could land on."

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