Dame Elizabeth Hoodless, chief executive of CSV, said that materials needed by volunteers to take part in the day might not make it to them in time for the annual event, which is being held on 31 October.
CSV is worried that its guides and promotional materials, such as T-shirts, balloons, certificates and chocolate, will remain undelivered if three days of strikes go ahead next week. Postal stoppages are due to occur on 22 and 23 October.
Last year, more than 60,000 people took part in Make a Difference Day. "It would be a tragic, unintended consequence if this work were jeopardised by an industrial dispute," said Hoodless.