Dog balloons
Dog-shaped balloons have been tied to Northern Line carriages on the London Underground@afrahrobleh2002

Carriages on London's underground network have been given a makeover by a mystery artist after balloon dogs were tied to handrails.

The brightly coloured balloons were first spotted on Monday and have so far only been seen on one line, the Northern Line which runs from Morden to Barnet.

Passengers have used social media to back the seemingly random phenomenon, saying it has brightened up people's moods.

But it could soon be the end of the line for the dog-shaped balloons.

A Transport for London spokesman said it had not received any official requests to tie the balloons onto handrails, adding any litter or additional items that are left on trains overnight are taken away by cleaners.