A riverboat stopped Kingston railway bridge from being hit by a 25ft barge after vandals untied it and released in into the river.
Teddington Royal National Lifeboat Institute (RNLI) was called out at about 4.45pm on Friday afternoon to reports of the released barge just upstream from Kingston railway bridge.
But the river boat owner, who is said to have been moored illegally, saved the day.
Andy Butterfield, from Teddington RNLI, said: “He recovered the barge and in doing so he went into the bridge himself and got his boat cut on the scaffolding.
“Had he crashed we would have been rescuing him as well as the barge.
“The barge had been securely tied off so someone had deliberately released it.
"Only a week before someone had released a barge with the owners still on it."
The barge is believed to belong to someone mooring close to John Lewis in Kingston town centre.
Kingston police confirmed they were called at 5pm to reports of a boat drifting down the Thames but it was dealt with by the coastguard.
Mr Butterfield said: “There were a few people watching. It was quite lucky. We didn’t have to do anything in the end.”
Got a story? Call the newsdesk on 020 8744 4244 or email firstname.lastname@example.org