The number of individuals forced to flatshare well into their 40s has risen dramatically, with figures showing a surge in middle-aged renters priced out of the property market and with little choice but to live with strangers.

Between 2009 and 2014, the number of flatsharers aged between 35 and 44 rose by 186%, according to Spareroom, the UK’s biggest flatshare website, while the number of sharers aged 45 to 54 went up by 300%.

Most are locked out of buying by soaring house prices, but also find that renting their own property is unaffordable. Average rents across Britain have gone up by 10.5% over the past year, far ahead of the rise in earnings, with the typical London rent now £1,558 a month, and close to £1,000 across much of the south of England. Tenants are typically handing over 50% of their salaries to landlords, rising to 72% in the capital.

Photographer Alex Forsey, 44, is typical of the new generation of flatsharers in their 40s. He recently moved into a house in Clapton, east London, with three other people in their 20s to 40s. He said: “The most affordable one-bed flat I was shown was £1,200 a month, and it was worse than a squat. The agent tried telling me it was a bargain.”

He described his flatmates as “lovely people, very warm and welcoming” but said flatsharing at his age left him anxious and unsettled. Suggestions that renters squander their cash on holidays and iPhones rather than diligently saving for a deposit on a home leave him fuming. Forsey said: “It’s absolute rubbish. I have not had a proper holiday in two years. I get quite well paid, but I’d have to put £40,000 down as a deposit for one of the tiny boxes that developers are putting up in London.”

Half of those renting in the UK despair of ever being able to afford a home of their own, according to a Post Office survey last week. Meanwhile, an Asda poll found that a third of renters have had arguments with their flatmates, with infuriating habits including not paying rent or bills on time, letting unwelcome cats into the house and the theft of underwear.

Rachel Churney, 41, started flatsharing after her job took her to London. Early in her career she had been able to buy in the north, but now found that not only could she not afford to buy in the capital, even a self-contained rental was out of the question.

Rachel Churney, 41, who lives in New Malden, south-west London, worries about the lack of security inherent in renting. Photograph: Martin Godwin/The Guardian

Adding to her problems, she wanted to take her labrador with her, and landlords almost universally reject applicants with pets. Eventually, Churney managed to find a two-bed flat in New Malden, south-west London, about 10 miles from the city centre. But to afford the rent, she had to find a flatmate. She said: “It means you have to go on to websites and find a complete stranger to live with.”

Her first flatmate was 10 years younger, from Spain and despite her being “a lovely person”, Churney found her age meant she was the one who took on the responsibility for organising things such as bills. She is also worried about the constant insecurity, compared with those who have bought their own home. She said: “I’ve been told by the landlord that they want the property back, and we have to move out in January. I’m probably going to have to almost bankrupt myself and get a place of my own. Even then, I know the maximum security I’ll have is one year.”

Campaigners blame a chronic lack of housebuilding in the UK. Dan Wilson Craw from Generation Rent said: “The failure of governments to build enough houses is forcing more people to make compromises in order to afford a roof over their head. For some, sharing is a lifestyle choice, but many others would prefer their own place, especially if they wanted to start a family. As long as house prices and rents keep rising, more of us will face years of arguments over whose turn it is to buy the loo roll.”

But others talk of the benefits of flatsharing, even into their 40s. Charlotte Ashton, 34, and Jannine Leadbetter, 43, share a two-bed flat in Chiswick, west London.

Ashton said: “I’ve shared all my adult life, and have been in my current flat with my lovely housemate, Jannine, for two-and-a-half years. We are both not in a position to buy as London prices, especially Chiswick ones, which are now £1m-plus on average, are just too high, and we’ve got caught in the trap of renting a beautiful flat with a direct Thames view, so buying (unless we won the lottery) would inevitably be a downgrade.”

Before moving in, the women had never met , but with her long experience of flatsharing, Ashton said she would know within five minutes if it was going to work. “What’s vital is that we each have our own space. We’ve never argued or even squabbled over anything. It’s all about respecting each other. But the real key is making sure you have a cleaner.”

Jannine Leadbetter, 43, and Charlotte Ashton, 34, right, at their shared flat in Chiswick, west London. Photograph: Graham Turner/The Guardian

She added: “I also enjoy the flexibility. At short notice I can just bugger off abroad if I want to.” Why is she not buying her own place? Ashton replied: “When I first moved in, the decent two-bed flats in Chiswick were £375,000. Now they are £495,000.”

Middle-aged flatsharing is not just confined to London, according to Easyroommate.com, one of the many websites springing up to handle the soaring demand for flatsharing. It said that among its advertisers seeking flatmates, the number aged over 40 is higher in Birmingham than London.

Campbell Robb, Shelter’s chief executive, said: “With our housing market out of control and rents sky high, it’s sadly no surprise that more and more people are finding themselves trapped in insecure flatshares well into their 40s.

“Government schemes like Help to Buy are only pushing up house prices even further, and not doing anything to help those who really need it. The autumn spending review is the government’s last chance to show it is serious about solving this crisis by investing in the genuinely affordable homes we desperately need, and giving renters back the chance of a stable home.”