Thousands of parents could miss out on their first choice of school for their children as a result of the postal workers' strikes.
The national deadline for applying for school places for September 2010 is at the end of the month. The deadline for London is 23 October.
The Communication Workers Union announced on Thursday its members would strike for two days next week in a dispute over pay, jobs and modernisation. Council leaders are worried that thousands of applications outlining preferred school places will be held up in the backlog of mail.
"The proposed Royal Mail strike could cause confusion and concern for families and could throw the school admission process into disarray," said Shireen Ritchie, who chairs the children and young people's board of the Local Government Association (LGA).
Applications received after the deadline are likely to be rejected by popular schools – meaning children will not be admitted if the school their parents prefer is oversubscribed.
The LGA is urging parents to either fill in the form online or get proof of posting before the deadline.
The first 24-hour national strike will see mail centre staff and drivers walk out on Thursday.