The president of the European Council said EU leaders should concentrate on practical measures such as reinforcing borders and a banking union.
Mr Tusk warned of "dramatic consequences" if Britain left the EU.
He urged Europe's leaders to match the energy and endeavours of those who wanted to break up the union.
Meanwhile, Spain's Prime Minister Mariano Rajoy has warned that the consequences of an exit vote in the UK's EU referendum later this month would be "very negative for British citizens".
In a speech in Madrid, he said people from the UK would lose their right to move freely, work and do business in the largest economic area in the world.
More than 300,000 Britons are resident in Spain, about 35% of whom are retired.