A man rescued a young boy who slipped through railings and fell into the River Thames on Tuesday afternoon.
Mark Maloney, 37, was at The Old Thameside Inn on Bankside when he heard the boy's mother screaming for help.
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He said his training for triathlons got him through the incident.
Speaking to the Standard, he said: "I do a lot of triathlons and lots of open swimming so I thought I could help.
"In the water I was panicking because I couldn't see him, I was screaming up to my mates 'where is he?' And suddenly his head popped up in front of me."
Mr Maloney said that even though he did an open water swim in an Ironman competition in the summer he was still struggling to keep afloat.
He managed to get them both to the bank and climbed a ladder out to the boy's mother's great relief.
Mark said he believed she was in shock and could barely speak as she tried to say thank you.
He added: "I was just so relieved to get the boy out and see he's ok. It's a sense of relief that I have never known before to give him back to his family just before Christmas."
Mark was visiting London from Newcastle under Lyme for work and was with colleagues at the time of the incident. His boss described his actions as "heroic" and added: "The kid was going under - you could just see the top of his head and hands as he was struggling to breathe. Seconds later and he would have gone."
According to the Daily Mail, an RNLI spokesman said: "RNLI Tower lifeboat was launched at 2pm to reports of a person in the water by the Thameside Inn.
"There was a strong, ebbing tide which can be extremely dangerous. En route it was called through to say the person was clear of the water but medical assistance was required.
"The lifeboat crew gave medical aid to a four-year-old boy who was reported to be fully conscious but wet and distressed. Both were handed over to paramedics for further medical assistance."
Mark hopes to keep in touch and hear how the boy is doing.