A London Underground train has hit an obstruction on the Bakerloo line, causing severe delays.
The line was suspended between Elephant & Castle and Piccadilly Circus, while the rest of the line was severely disrupted.
Transport for London (TfL) said the tunnel had not collapsed and that no-one was injured.
It comes as maintenance workers on three lines are on strike but the Bakerloo Line is not one of them.
Reports on Twitter and by the Rail Maritime Transport Union (RMT) said the tunnel had collapsed.
The union later said the incident had been caused by the train hitting a bulge in the tunnel wall, which had been caused by rain water.
'Irresponsible'
London Underground (LU) said it believed something may have fallen off a tunnel wall and that there was no risk of the tunnel collapsing.
Its engineers are assessing the situation.
Nigel Holness, LU's operations director, said: "There is no tunnel collapse in the Bakerloo line, nor is there any flooding or any injuries. It is wrong and irresponsible to suggest otherwise."
He added: "The problems this morning are not in any way connected with the industrial action taken by some maintenance workers on the Jubilee, Northern and Piccadilly lines, which has had no impact since the strike began."