The line - known as The Drain - was suspended for most of yesterday and this morning because drivers complained they were having problems seeing properly.

The line was closed at 7.50am this morning and reopened at 11.12am.

The dust was a result of weekend rail grinding works by the controversial Tube maintenance firm Metronet.

LU managing director Tim O'Toole said Metronet had not done enough dust suppression, leaving a "terrible mess".

The busy line only reopened in September after five months of upgrade works which commuters were promised would make it more reliable.

Mayor Ken Livingstone said Metronet's contract may be scrapped if the firm's work did not improve next year.

Metronet today apologised to LU and passengers for the suspension of service.

A Metronet statement said: "As part of its planned operations, Metronet last weekend undertook work to grind the rails in the two tunnels of the Waterloo & City line.

"This work is prudent and responsible and is typically undertaken after completion of a major upgrade. It follows best practice and is designed to ensure that the life of the rail is preserved well into the future.

"As a result of this exercise a lot of dust was created and this has taken some time to remove. We had a team of 30 cleaners on site who used industrial vacuum cleaners and magnetic wands to eliminate the dust."