There is no service between Euston and Watford Junction while engineers fix a signal failure in the Stonebridge Park area.
The Bakerloo line is also affected and is part suspended.
There is no service between Stonebridge Park and Harrow & Wealdstone and there are minor delays between Queens Park and Stonebridge Park caused by the same signal failure problem.
For full service updates go to the Transport for London website.
Minor delays on Bakerloo Line between Queens Park and Elephant And Castle due to signal failure at Charing Cross.
The Bakerloo Line is now running with minor delays between Queen's Park and Harrow & Wealdstone due to an earlier fault with the track at Marylebone.
Service suspended on Bakerloo Line Southbound between Paddington and Elephant And Castle and severe delays on the rest of the line due to fault with the track at Marylebone.
Nigel Holness, London Underground's operations director, said:
RMT general secretary Bob Crow said the child, who strayed onto tracks on the Bakerloo Line, could have been electrocuted.
A tube driver has averted a "potential tragedy" by rescuing a 12-year-old boy who had strayed onto tube tracks on the Bakerloo Line.
The Rail, Maritime and Transport Union says the child narrowly missed the 430-volt live rail, after jumping onto the tracks from a train that had pulled into sidings following its final passenger stop.
It's the third case in as many weeks of a child getting on to tracks on the Underground. Two weeks ago, an eight-year-old was led to safety by a cleaning contractor after being spotted on tracks near Stanmore. Last week a 12-year-old girl was hit by a tube train in Brixton.
Minor delays on Bakerloo Line due to earlier faulty train at Baker Street.