Cylinders of oxy-acetylene gas, which are about 4ft tall and usually used on building sites for welding, were reportedly used in the failed robbery at Highgate Post Office, in Archway Road, in the early hours of Friday.
Despite the explosion, the section of the ATM machine that holds bank notes was not blown open and the raiders escaped empty-handed.
Postmaster Michael Darvill, 75, said: “I’m told that it was done by gas and it was oxy-acetylene gas.
“They set fire to it which caused an explosion – it’s hugely volatile.
“They hoped they would blow the back doors off the safe inside the ATM machine. It’s important people realise there is a safe inside.
“They left a lot of debris around the place but no money at all was taken. They didn’t get away with a thing, which is very good, but it’s caused an awful lot of damage and cost.”
Flying Squad detectives were called to reports of a robbery at about 4.20am but the suspects had fled the scene by the time officers arrived.
Police said debris found was consistent with a small explosion.
Detectives are investigating whether CCTV footage of the incident is available.
Mr Darvill was told a police patrol car passing at the time had CCTV equipment on board.
No arrests have been made.
The sub-postmaster has owned Highgate Post Office for 50 years and said it is not the first time the shop has been targeted.
“This is the trouble with being a sub-postmaster,” he said.
“It always has been, because we are the money shop in the high street and people think we haven’t got security. But we do have a lot of security these days.
“I’ve been a sub-postmaster for 60 years this Christmas and I think it’s been the sixth or seventh attack I’ve had.
“I suppose this is the worst, and yet it’s the best because nobody was injured.
“As far as I’m concerned no money at all was taken and had money been taken, so what – money can be replaced, people can’t.”
He said he felt lucky no one was injured in the robbery.
“Quite honestly you will find any builder is always very, very wary of oxy-acetylene,” he said.
“I have absolutely no idea how they did it and I think that’s why the police were worried about it because people will eventually get killed, so they want to nip it in the bud.”
Anyone with information is asked to call police on 101 or Crimestoppers anonymously on 0800 555 111.