The rise in the number of married couples who are choosing to live apart is contributing to a severe housing shortage, a report suggests today.

The document from Shelter, the homeless charity, seen by The Times, predicts that by 2026 there will be 4.5million more households but one million too few homes to accommodate them.

Seventy per cent of all the new homes needed by 2026 will be for single people who will not marry, more elderly people living longer at home and children wishing to leave their parents earlier.

Today's report estimates that 1.1million women and 800,000 men who have regular partners are living in separate homes. The couples have decided to keep their own homes or to move to separate ones for tax or lifestyle reasons or because their relationships have deteriorated. The discrepancy between the numbers of men and women is likely to be because of under-reporting by men or because men are more likely to have more than one regular partner, researchers suggested.

The “living apart together” relationships have been well documented among creative couples, including Woody Allen and Mia Farrow, Helena Bonham Carter and Tim Burton, and Margaret Drabble and Michael Holroyd. The report suggests that it is becoming increasingly common among middle-aged, middle-class households that can afford to run two homes.

The Shelter report claims that the credit crunch and the overall increase in households will lead to serious housing shortages in the next two decades. Gordon Brown's target of three million new homes by 2020 is based on 200,000 homes a year being built now and 240,000 homes per annum being built by 2016. Only about 100,000 houses a year are being built, however, with little prospect of significant increases in the next two years.

Shelter predicts that there will be a shortfall of one million homes by 2026, including a 300,000 shortfall in affordable and social rented homes.

The number of married couple households is expected to fall from 9.4million in 2006 to 8.9million by 2026, while the number of cohabiting couples will rise from 2.2million to 3.4million. The greatest predicted increase will be in one-person households, from 6.8million in 2006 to 9.9million by 2026.

Adam Sampson, Shelter's chief executive, said: “If the slowdown in output continues for more than a couple of years, it is unlikely that the Government's target to deliver two million additional homes by 2016, increasing to three million by 2020, can be met. The housing market downturn has created a different environment that needs higher levels of public subsidy than at present.”

One Plus One, an organisation that helps families and couples, said that changing patterns in relationships would play a big role in single-occupancy households. “As couples increasingly delay marriage until their early thirties, more people are able to afford to live on their own and remain single well into their professional lives,” a spokesman said.

CASE STUDY: Ruth Gledhill

My husband and I met when relatively young, we had a child, and married late. This meant our quotidian habits were formed, pretty much set in mortar in fact, by the time that the question of setting up home together came up.

Neither of us really wanted to change our domestic habits, and neither of us has. We had both shared homes with previous spouses, experiences we did not want to revisit upon each other.

It still amazes me that he leaves his dirty socks on the floor in my house and never in his own. But as it's my house I have no compunction about throwing them in the bin.

I somehow seem to have landed myself with all the children - including his 25-year-old adult returner - and the au pair as well as the cat. But coming from a large vicarage family with a menagerie of parrots, cats, dogs, horses, rabbits and foster children I love the feeling of chaos that can threaten to overwhelm us.

Our arrangement means that we all get the best of both worlds. I have the Wii, the PlayStation, the beat of the pop star in the middle bedroom, the piles of ironing that even our long-suffering cleaner can't get to the bottom of, the little garden with the gazebo that is about to be turned into an art studio. He has the peaceful space away from aircraft noise on the top of Richmond Hill, the fabulously large garden for sitting and reading in.

I don't care much about furniture or even clothes, but books are something else. We both have too many to fit into either house. Selling one would mean having to buy somewhere really humongous, a grand house neither of us would feel entirely comfortable inhabiting.

Then there is inheritance. At the moment it is simple. His kids get his house, mine gets mine.

We spend most of our time together, in my house. We use his place as an office when one of us is working from home. At any time we know that if either one of us gets fed up, we can simply run up or down the hill and escape. That has never happened yet. We can also use it as a couple, to get away from everyone else. That does happen occasionally, often enough to think that this is a marriage that might last.

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