Net migration is causing the UK to grow at the rate of a city the size of Cardiff every year, according to the leader of UKIP in the Welsh assembly.
Neil Hamilton said the country "cannot cope with the scale and the speed of current immigration".
Net migration to the UK rose to 333,000 in 2015, according to Office for National Statistics estimates
Plaid MP Liz Saville-Roberts said the main concern was not of EU migrants, but of migrants from further afield.
Last year's net migration estimate - the difference between the number of people coming to the UK for at least a year and those leaving - was the second highest figure on record.
The figure for EU-only net migration was 184,000, equalling its record high.
Mr Hamilton, speaking on Thursday night's BBC Question Time in the Welsh capital, said: "We're adding a city the size of Cardiff to the national population in the UK every single year and this is set to continue for the foreseeable future; a million extra people every three years.
"How are we going to cope? How are our public services going to cope with the speed of change?"
But Ms Saville-Roberts, also on the panel, said that the main concern was those who came from beyond the EU.
"Evidently, when we look at the economy, and unemployment, on security, aren't we, surely, better off working with our neighbours in Europe?" she said.
"The main concern for migration isn't the EU migrants which we're discussing here, it's the migrants who are coming from beyond Europe; sub-Saharan Africa; from the middle east, from conflict zones, how are we dealing with that?"