Heavier trains are wearing out tracks across south west London, the rail regulator says.

The Officer of Rail Regulation says some heavier trains brought in by Southern and South West Trains are causing a new kind of wear on tracks, while Network Rail has blamed their stiffer suspension for delays.

"The new trains are heavier and have stiffer suspensions than their predecessors so Network Rail is working to deliver a maintenance regime that is appropriate to the conditions," a Network Rail spokesman said.

He said Network Rail would make sure new trains would be designed to be safer and lighter.

The ORR said the companies would have to increase maintenance to repair the damage.