London councils have granted property developers planning permission to build more than 26,000 luxury flats priced at more than £1m each, despite fears that there are already too many half-empty “posh ghost towers” in the capital.

Builders are currently constructing towers containing 7,749 homes priced between £1m and £10m, and have planning rights to build another 18,712 high-end apartments and townhouses, the Observer can reveal. Politicians and housing campaigners said the figures show councils are prioritising the needs of the super-rich over those of hardworking young Londoners.

The boom in developments of luxury flats, which often include private cinemas, gyms, swimming pools and concierge facilities, comes as the capital faces a growing crisis in the availability of affordable housing, with nurses, police officers and other essential workers struggling to get on to the housing ladder.

Research shows that a fifth of aspiring first-time buyers have moved in with their parents to save money, and a quarter of them will need to stay there for at least five years to amass enough for a deposit. The proportion of English first-time buyers who rely on help from families and friends for their deposit has increased from 22% in 1996 to 29% in 2016, according to the government’s English Housing Survey.

Anne Baxendale of Shelter said: “The UK is in the grip of a housing crisis and nowhere is this more apparent than in the capital – and these luxury developments are certainly not the types of homes most Londoners need. The government must close loopholes which make it easy for developers to build high-priced homes that are way out of reach of ordinary families, rather than the affordable ones most people actually need and can afford.”

David Lammy, the Labour MP for Tottenham, said the figures “reveal a travesty being played against the working class and young Londoners”.

“The public keep being told we are building more affordable housing, and people can see cranes up all over London,” he said. “But this shows that councils are prioritising the fancies of overseas millionaires and billionaires before the needs of hardworking young Londoners.”

Just 6,423 affordable homes were built in London during the 2016-2017 financial year (the latest figures available), a 5% decline on the previous year and a big drop from the 19,622 built in 2014-15.

Siân Berry, a Green party London assembly member and chair of the housing committee, said: “These £1m flagship flats are a waste of space, they are designed for people as an investment they might visit occasionally.”

The research – for private bankers at Coutts, shared exclusively with the Observer – shows that despite the existing glut of luxury homes, more are planned for London than ever. The data shows huge numbers have been granted planning permission in the capital, with more than 6,500 £1m-£10m homes proposed in the redevelopment of Earls Court exhibition centre and surrounding area.

South of the river, 1,689 luxury homes are being built in the Battersea, Clapham and Wandsworth area on top of the dozens of towers already constructed in the Nine Elms development near Battersea power station. Wandsworth council has granted permission for another 2,314 homes which will be priced at more than £1m each.

The council has allowed the developer of the Battersea power station site to build just 386 affordable homes in the 4,239-home redevelopment. Only 9% of the homes will be affordable, far below London mayor Sadiq Khan’s 35% affordability target for all new large developments.

Khan said he was “furious” that Wandsworth council had allowed the Malaysian-backed Battersea Power Station Development Company to cut down its original promise of 636 affordable homes in the £9bn transformation of the London landmark. The developer had complained that falling sales of luxury homes would knock its profit margin on the redevelopment, which will also include a new UK headquarters for Apple.

The Coutts figures, compiled by housing data service LonRes, show that developers are pushing ahead with the vast number of expensive new flats despite failing to sell more than half of the 1,900 luxury homes they built in London last year. A record 3,000 units priced at more than £1,500 a sq ft – which works out at roughly £3m for a three-bedroom flat – are currently on the market. The average price per sq ft of homes across the UK is £211.

Coutts, which will only allow customers to open an account if they can prove they have at least £1m in assets, said it was advising clients to be cautious about buying luxury homes with fears that the market was hugely oversupplied and prices could crash.