One in three children in London missed out on a place at their top choice of secondary school.
In some boroughs, more than 40% of children failed to secure a place at their first preference.
Just 66% of pupils who applied to start at a secondary school in the capital received an offer from their first preference school, compared to 68% last year, according to data from the Pan London Admissions Board.
Across London's 33 boroughs, secondary schools received 93,727 applications this year – a 0.4% increase on last year.
A breakdown by London borough shows significant differences in the proportion of families securing their top choice.
Hammersmith and Fulham had the lowest proportion of children getting their top choice at 57.5%, and in Greenwich just 59.9% secured their first preference.
Havering had the highest proportion of first preferences at 81.2%, followed by Waltham Forest where 79.9% secured their preferred school.
Covid-19 disruption in the autumn term meant delays to selective school tests.
Families are normally informed of their child’s test score for a selective school before the admissions deadline and can take this into account when making their secondary school applications.
But this year many grammar schools could not confirm places until after the deadline due to the coronavirus pandemic.
Cllr Elizabeth Campbell, London Councils’ executive member for schools and children’s services, said the number of secondary school applications in the capital have “remained stable” compared to last year.
She added: “However, we recognise that the Covid-19 pandemic has caused delays to some selective school tests and this will have contributed to the slight decrease in overall preference figures.
“London boroughs are working with local schools to respond to demand across the capital for school places and will continue to work diligently with teachers to ensure pupils have the emotional and social support they need to transition to secondary education as smoothly as possible.”