Environmental groups have called on Amsterdam city council to abandon plans to release 150,000 balloons into the air at the end of celebrations to mark the investiture of king Willem-Alexander.

The city’s mayor Eberhard van der Laan says the balloons are biodegradable, but the groups say it still takes considerable time before the balloons break down. In addition, the strings attached to them pose a hazard to birds and other wildlife.

The protest letter was signed by groups as diverse as Greenpeace and the government’s own forestry commission Staatsbosbeheer.