Half of secondary schools in England are over-subscribed, with the best being the hardest to get a place at, research suggests.
In some areas of the country, the vast majority of secondaries rated outstanding by Ofsted have more families applying than spots available.
The analysis was published by 192.com's FindASchool service just days before the October 31 deadline for parents to submit applications for children starting secondary school next autumn.
It comes amid growing concerns about a squeeze on places, and the impact a bulge in pupil numbers is having on schools.
The figures, based on Freedom of Information (FoI) requests and data gathered from schools and councils, show that 49.8% of all secondary schools across the country are over-subscribed - meaning that they have to turn some children down.
This rises to almost two thirds - 63% - among those judged to be good or outstanding by inspectors.
A breakdown of the statistics suggests that parents' chances of gaining a place at an outstanding secondary depend heavily on where they live - in the West Midlands, 96.8% of schools are over-subscribed, while in the East of England the proportion is 94.3% and in London 92.4% of secondaries have more applications than places available.
Schools have to publish details of how they will award places fairly in the case of over-subscription. This often includes allocating places based on a particular "catchment" area, or prioritising the brothers and sisters of pupils already at the school.
Around one in six children in England missed out on a place at their top choice of secondary school this year, according to Government statistics.
In total, 84.1% of 11-year-olds across the country were given a spot at their first preference, compared with 84.2% in 2015.
Separate Department for Education (DfE) figures released in July suggest that the secondary school population is projected to continue increasing to 3.04 million by 2020, and peak at 3.33 million in 2025.
The number of over-subscribed schools is likely to continue to grow unless more places are added, FindASchool warned.
Spokesman Dominic Blackburn said: "The data we collected shows an alarming trend that secondary schools are becoming increasingly difficult to secure a place at.
"Clearly, not enough is being done to increase the number of school places and give parents a genuine choice of school.
"The days of assuming you can just send your children to the best local school where they will get a place are over."
"FindASchool lets parents easily see which schools they are likely to be able to get a place at, and check that they meet the criteria needed to get in - meaning that parents are not wasting their application preferences on schools they are unlikely to secure a place at."
Justine Roberts, chief executive of parenting website Mumsnet said: "Lots of parents believe getting into the right school has a big impact on their children's life chances and happiness, so it's a much-discussed topic on Mumsnet.
"Come offers day, many are delighted with the school they've been allocated, but others - particularly in areas where popular schools are over-subscribed - can be horribly disappointed.
"Among parents in areas that are particularly badly affected, it seems to be widely accepted that many will bend the rules as far as they can go without snapping."
A Department for Education spokesman said: "These figures do not paint the full picture as they do not take into account the new places that will be created in the future, nor the surplus places that are already available. Over nine in 10 parents in England received an offer at one of their top three preferred secondary schools this year.
"We are delivering good quality school places to ensure every child has an excellent education that allows them to reach their full potential. Nearly 600,000 additional pupil places were created between May 2010 and May 2015, in response to the growth in the school age population, and we are investing £7 billion in new places up to 2021."