The Northern Line extension, which opened up vast swathes of south London to commuters, is celebrating its 75th birthday this week.

To celebrate, a birthday cake was cut yesterday and leaflets about the line were handed out to passengers at stations.

When it first opened in 1880, the Northern Line ran between Stockwell and King William Street near Bank. Balham, the last station on the

extension to be completed, opened on December 6, 1926.

At that time the cost of a weekly pass between Morden and Charing Cross was just 27 pence meaning that hundreds of people living in the suburbs were able to travel to jobs quickly and easily.

All the stations on the extension including Clapham South originally due to be called Nightingale Lane Tooting Bec, Tooting Broadway, Colliers Wood, South Wimbledon and Morden, were designed by Charles Holden who also designed London Undergrounds circle and bar logo.

The line now serves 50 stations and runs for 36 miles. Last year 204 million journeys were made.

A 20-year £1billion improvement campaign is underway and work is currently being carried out at Morden. All the other stations have already been modernised and an entire fleet of new trains was introduced in the late 1990s.

Tube managers say the new Public Private Partnership (PPP) will mean another huge injection of cash into the line, resulting in a more reliable and frequent service. However, the proposed PPP has been criticised by transport watchdogs wary of any attempts to privatise the system.

Bill Tucker, Northern Line group station manager, said it was now the top performing line in the Underground system.

He added: The opening of the extension in 1926 changed London by creating easy and affordable access into the capitals centre.

In 75 years London has developed radically in terms of population growth and transport needs which we continue to address as much as possible.