Labour mayoral candidate Sadiq Khan says he will work "tirelessly" to extend the Bakerloo line beyond Elephant & Castle if he is elected to City Hall in May.
Mr Khan's manifesto – published on Wednesday – pledges to "work to secure the proposed Bakerloo line extension to Lewisham and beyond".
"A 21st century city needs a 21st century transport system and that's why, if elected, I will get straight on with planning London's next round of transport improvements," said Sadiq Khan.
"Areas of London like Lewisham have been disadvantaged by not being part of the tube network.
"Extending the Bakerloo line to Lewisham and beyond will make a huge difference to Londoners in that part of our city, allowing them quicker, easier access to the rest of the city.
"I will work tirelessly to secure that proposed Bakerloo extension."
The other frontrunner in the contest to be the next Mayor of London – Conservative Zac Goldsmith – has yet to publish his full manifesto. His 'action plan' refers to "growing the rail network" but omits any mention of tube extensions.