Labour's Sadiq Khan has enraged Brexit campaigners by flying the EU flag alongside the Union Jack outside City Hall. The new Mayor of London said he made the move to symbolise his pro-remain position on the 23 June referendum.

Khan wants the UK to avoid breaking away from Brussels at the historic ballot because he claims London benefits from NHS workers from EU countries and membership of the 28-nation bloc has a positive effect on the capital's environmental well-being.

But Ukip's group leader on the London Assembly attacked Khan, who succeeded Eurosceptic firebrand Boris Johnson, over the decision.

"I was amazed today to discover that one of the first acts of Sadiq Khan has been to order the flying of the EU flag from City Hall. He just couldn't wait to display where his true loyalties lie. It is extraordinary," Peter Whittle declared.

"Luckily there is an opportunity to make sure that the EU flag is never flown outside City Hall again and that is a Leave vote in the forthcoming EU referendum.

"It is also extraordinary that the English flag has not been flying from City Hall. London is the capital of England and we know full well that the Scottish and Welsh flags fly from their capital cities. So why not England? Seems to me to be most inappropriate."

Elsewhere, Khan officially announced the launch date for London's long-awaited Night Tube. The 24-hour service will be rolled out on the Central and Victoria lines on Friday 19 August, with services on the Jubilee, Northern and Piccadilly lines to follow in the autumn.

Khan, the capital's first Muslim mayor, said: "The night tube is absolutely vital to my plans to support and grow London's night-time economy – creating more jobs and opportunities for all Londoners. The constant delays under the previous mayor let Londoners down badly."

City Hall, London
The EU flag alongside the Union Jack outside City Hall Twitter/Sadiq Khan