David Cameron has said a poster showing migrants under the headline "Breaking Point", unveiled by Nigel Farage in support of Brexit, was "irresponsible".
Appearing in a special referendum edition of the BBC's Question Time programme, the Prime Minister also reiterated his view that the leader of Isis would prefer if Britain voted out.
His appearance on the programme came as:
In the final Sunday of the campaign both leave and remain sides were out in force trying to persuade those who have yet to make up their minds to support their cause. Campaigning was suspended on Friday and yesterday following the murder of Jo Cox.
George Osborne stepped up warnings of the threat to the economy saying that the damage could be "quite a lot worse" than even the International Monetary Fund (IMF) was predicting with hundreds of thousands of jobs at stake.
For the Leave camp, Justice Secretary Michael Gove insisted Britain would be able to cope with "whatever the world throws at us" outside the EU.
However the Ukip leader Nigel Farage admitted the campaign had lost momentum following its suspension in the wake of the alleged murder last Thursday of Mrs Cox in her West Yorkshire constituency.
"We did have momentum until this terrible tragedy. It has had an impact on the whole campaign for everybody. When you are taking on the establishment, you need to have momentum," he told ITV's Peston on Sunday.
Amid heightened sensitivity over the way immigration has featured as an issue in the campaign, Mr Farage found himself under attack from senior figures on both sides over a controversial Ukip poster showing migrants queuing to get into the EU under the slogan "Breaking Point".
Mr Farage rejected claims that he was stoking up hatred, complaining that he was the one who was being targeted.
"I think I have been a politician who has been a victim of it," he said. "When you challenge the establishment in this country, they come after you, they call you all sorts of things."
With immigration set to be one of the key issues in the remaining days of the campaign, the official Vote Leave campaign seized on an admission by Labour leader Jeremy Corbyn that there could be no upper limit as long as Britain was in the EU.
"I don't think you can have one while you have the free movement of labour," he told The Andrew Marr Show.
Concerns about immigration have widely been blamed for driving traditional Labour voters to swing heavily towards Leave.
The pro-Brexit Labour MP Gisela Stuart, who chairs Vote Leave, said his comments exposed the inability of the Remain camp to control numbers coming into the UK.
"They have no plan for how we will fund the NHS so it can cope with the extra pressures that staying in the EU will create," she said.
"They have no plan for where we will build the extra houses and they have no plan to help people who will see further pressure on their pay packets."
