Rush hour commuters are facing severe disruption on the Overground line between Chingford and Liverpool Street.
Trains could be cancelled, delayed or journeys cut short due to a signalling fault in the Bethnal Green area which is blocking the line between Hackney Downs and Liverpool Street.
Operator Abellio Greater Anglia has said disruption is expected until at least 9.30am.
Tickets are being accepted on the Victoria Line from Walthamstow Central, Tottenham Hale and Seven Sisters, as well as on the Central line from Stratford to Liverpool Street.
A spokesman said: "Customers are advised that journey times will be extended by at least 10 minutes.
"Network Rail engineers are on site to rectify the fault.
"There is no firm estimate yet of how long disruption will last but it is likely to continue until at least 09:30am."
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