TUBE passengers have been left frustrated after the Waterloo and City line was suspended this morning - just a day after re-opening following five months of upgrade works.

Line operators Metronot promised the £40 million works would mean a more reliable service, but commuters experienced delays and suspensions through out the morning due to signaling failures and a stalled train.

The line was suspended while Metronet investigated the signaling fault at Bank Tube station, in central London.

London Underground managing director Tim O'Toole said the line should be running reliably following the multi-million pound upgrade.

"The Waterloo & City line is experiencing problems due to signal failures this morning," he said.

"Following any closure for engineering works, especially a long-term closure, it is important that the line is reliable when it returns.

"We are demanding that Metronet address this issue immediately and ensure that these signal failures do not persist."

The line, nicknamed "The Drain", ferries over 37,000 passengers a day between Waterloo and Bank, without any intermediate stops.

The service was due to resume on September 1, but work by maintenance firm Metronet overran. The company will be fined hundreds of thousands of pounds for the delay.

A Metronet spokesman apologised to passengers.

"Metronet tested the line extensively before it reopened and test trains were running from August 23," he said.

"We're working as fast as possible to fix this problem and apologise to passengers for the delays.

"The indications are that a faulty signalling component at the Bank end was at fault. This component has been renewed and the system is now holding up."

Over the past five months, workers replaced the entire track along the 1.5-mile line and fitted new communication systems.

The five four-car trains, made in 1992, were refurbished with CCTV and London Underground's red, white and blue colours.