Cracks on the underside of trains were a key reason for recent delays on the District line, a new report has revealed.

The London Assembly investigation found the line, which connects underground stations in Wimbledon, Southfields and Putney to central London, was one of the four worst performing in the capital in 2010/11.

It found 4,806,000 customer hours were lost on the line over that period - and called for a 24 per cent reduction in this figure by next year.

The report, published today (Tuesday, September 13) by the assembly’s transport committee, said cracks in the equipment on the trains that collects power from the rails was the main problem on the route.

It found 3,908,000 customer hours were lost on the Northern Line, which passes through Morden, Colliers Wood, Tooting, Balham and Clapham.

The committee called for an 11 per cent reduction in this figure.

Committee chairman, Caroline Pidgeon, said Transport for London was responding to problems with its trains and operating systems.

But she said: “Whilst delays on the tube have reduced in the last eight years, last year was marked by the longest period of sustained period of poor reliability since the start of the upgrades [eight years ago].”

Mike Brown, managing director of London Underground, told the BBC some disruption was inevitable during the biggest upgrade in the network's history.

But he added: "Londoners rightly expect reliable journeys, and we are completely focused on making sure that we deliver that."