Delays on the London Underground caused by over-running engineering works have been described as "unacceptable".
The late-running work by maintenance company Tube Lines at Neasden in north west-London caused delays on two lines.
Services were suspended on the Metropolitan Line and the Jubilee Line during the morning rush-hour.
A spokesman for London Underground said it was "frustrated by this unacceptable performance". Tube Lines apologised to passengers for the delays.
Services were suspended on Friday morning on the Metropolitan line between Harrow-on-the-Hill in north-west London and Aldgate in the City of London.
Safety critical work
There were also no trains running on the Jubilee line between Stanmore and Willesden Green in north-west London.
Huge queues built up at Harrow - a busy interchange - with Tube passengers attempting to crowd on to main line services run by the Chiltern train company.
There is still disruption on both lines.
London Underground spokesman Howard Collins said: "We are extremely frustrated by this unacceptable performance by Tube Lines, which has caused disruption to thousands of our passengers' journeys this morning. We apologise to our passengers for the disruption."
A Tube Lines spokesman said: "Tube Lines apologises to passengers for delays encountered this morning. We had to complete safety critical work last night which was particularly complex.
"We are working with London Underground to ensure that this type of delay does not happen again."