Almost 4,000 London children have been rejected by every secondary school they applied to, figures show today.
It means five per cent of 11-year-olds are without the offer of a place, or have been allocated to the most unpopular schools.
More than one in three children failed to get into their first-choice school and the crisis will worsen in years to come, said London's admissions system chief Helen Jenner.
"Undoubtedly there are some very, very worried families," she said.
A population "bulge" due to hit secondary schools in the next three years will see the situation deteriorate further, she added.
Families applying for a place can list six schools in order of preference. Today's figures from the pan-London admissions board show 34 per cent missed out on their first choice - slightly better than last year when 35 per cent were rejected.
Five per cent of children were rejected by all six choices, compared with seven per cent last year.
Mrs Jenner, who chairs the board, said there are enough secondary school places for every child to get one, but they will not necessarily get their choice. She added: "There is an absolute need to build more secondary schools. If we don't, in three or four years time there will be a huge problem because there has been a huge population growth. We won't have enough places."
She advised parents: "Schools are important but support from families is more important in how a child succeeds. Every school in London has some children at least that do very well. With parental support every child has a chance to do well."
Competition for places at leading grammars and popular comprehensives is believed to be particularly high this year as the recession prompts many families who would otherwise have sent their children to private schools to consider state education.
Many academies and free schools are hugely oversubscribed, with nine pupils vying for each place at the most popular.
This year 88 per cent of children got a place at one of their top three schools, compared with 87 per cent last year.
'One shot to change our girl's life' - Andrew Neather
I felt a surge of relief when my wife told me our eldest daughter has got a place in September at her secondary school of choice. It's a good local comprehensive and the nearest to our home in Southwark: in truth, I wasn't that worried.
But many parents were. It feels to us that you're unlikely to get a place at oversubscribed schools unless you make them first choice. Partly for that reason, we did not bother to list other choices for our daughter: one shot that could determine so much in her life.
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For those with children in Year 6, it has been a subject of near-obsession for months. Last June I asked one local mother where they were going on holiday. "We're not," came the reply, because her son was studying so hard for the 11+ and private school scholarships.
Friends were recently fretting that the alternative to their daughter's first choice was so bad, if she didn't get in, they'd have to go private, which they can't afford.
Meanwhile one of my daughter's classmates who lives 10 minutes' walk from us has no offer at all. No parent should have to face this uncertainty.
