A group that wants lower immigration has forecast if Britain votes to remain in the EU, net migration will exceed 250,000 a year for at least 20 years.
The Migration Watch report said a "cautious estimate" for annual net migration by 2035 was 265,000, with 60% of migrants coming from the EU.
It said there was complacency about the impact of rapid population growth which it said "could change the UK forever".
Remain said leaving the EU and damaging the economy was not the answer.
Immigration has been a key battleground in the referendum campaign ahead of the vote on EU membership on 23 June.
Net migration to the UK rose to 333,000 in 2015, according to Office for National Statistics estimates released in May - the second highest figure on record - figures which the Leave campaign argue illustrates the inability of the UK to control migration while being in the EU.