A reduced service has been restored to all Northern line stations, but with severe delays, London Underground (LU) said today.

Tube bosses expect the service to improve later today, a spokesman said. "We hope to operate a good service tomorrow."

London's busiest Tube line, which carries 660,000 passengers on a week day, was closed last Wednesday over safety fears.

Drivers refused to work after the fifth incident in which emergency "trip-cock" brakes failed to stop a train that passed a red signal.

All stations on the line were re-opened this morning, two days ahead of schedule.

LU initially said today's trains would only run on central parts of line. Service across the entire line would not be restored "before Wednesday", LU chief operating officer Mike Brown said yesterday.

But engineers have worked around the clock to replace the faulty brake parts on all 106 trains.

On Friday ten trains with improved parts were "extensively tested" to ensure that there would be no further problems with the trip-cock brakes, LU said.

Health and safety representatives of trade unions agreed that the trains were safe.

"We have worked extremely hard to resolve the problems with Northern line trains as quickly as possible," Mr Brown said. "We needed to ensure that trains were 100% safe to operate before we recommenced services."

In an unprecedented move, LU used special powers to oversee the work of maintenance firm Tube Lines and its sub contractor Alstom.

Passengers should avoid the line until there are no more delays, LU said. Replacement bus services are still running.