Hundreds of rush-hour Tube passengers were trapped in sweltering heat in a tunnel after a train broke down.
An investigation has been launched
About 800 commuters were stuck in temperatures of more than 30C when the Central Line train malfunctioned between Bank and St Paul's station.
Some waited for more than an hour before being led to St Paul's station by walking through another train.
A Tube spokesman said the train broke down at around 0830 BST with some commuters stuck until 1000 BST.
"London Underground immediately instigated procedures to get people off the train," he said.
'Hot down there'
"Because the train would not move, we brought another train to the defective one and took people off in two groups."
He added: "It does get hot down there and it does get hot on stalled trains."
An investigation is being carried out by private maintenance firm Metronet.
The incident is thought to have been caused by a broken airline which affected the train's brakes.
There were no injuries and the Central Line re-opened at about 1000 BST.