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Check Realtime travel news for the latest service updates
We have been working 24 hours a day to get services restored
A reduced service is now running to all Northern line destinations.
Severe delays are occurring on the line but London Underground (LU) anticipates that the situation will improve during the course of the day.
Rail replacement bus services will remain in operation to connect with other LU lines.
Until a good service is in operation, customers should avoid the Northern line if at all possible and seek alternative Tube, bus and rail routes.
LU tickets will be accepted on all alternative services.
Nigel Holness, LU Service Director, said: "We have been working 24 hours a day to get services restored on the Northern line and were determined to do so as quickly as we could.
"We originally anticipated that services would not be completely restored across the entire Northern line until Wednesday.
"Tube Lines were able to make the modifications to all Northern line trains quicker than they had originally expected, and LU managed to get sufficient numbers of drivers briefed on modification procedures to allow us to operate services to all Northern line destinations this morning.
"Severe delays are likely for much of today but we hope the situation will improve during the morning and afternoon.
"We hope to operate a full service tomorrow."
For 24-hour information on public transport in London, please call: 020 7222 1234.
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