David Cameron has been criticised for his description of migrants trying to reach Britain as a "swarm".
Asked about the Calais crisis, he spoke of "a swarm of people coming across the Mediterranean, seeking a better life, wanting to come to Britain".
Labour's Harriet Harman said "he should remember he is talking about people and not insects" and called the use of "divisive" language a "worrying turn".
Hundreds of migrants tried to enter the Channel Tunnel overnight.
Thousands of migrants have been trying to reach the UK from Calais this week, with nine people killed attempting to cross the Channel in the past month.
Mr Cameron, who is on a tour of South East Asia, warned that illegal immigrants would be removed from the UK, saying Britain would not become a "safe haven" for migrants in Calais.
He said the French had deployed an additional 120 police to the French port and the UK was investing in fencing and security measures.
He told the BBC "everything that can be done will be done to make sure our borders are secure and make sure that British holidaymakers are able to go on their holidays".
Speaking to ITV News, the prime minister said the situation in Calais was "very testing".
"I accept that, because you have got a swarm of people coming across the Mediterranean, seeking a better life, wanting to come to Britain because Britain has got jobs, it's got a growing economy, it's an incredible place to live," he added.
Ms Harman, Labour's acting leader, told the BBC: "He should remember he's talking about people and not insects and I think it's a very worrying turn that he appears wanting to be divisive and whip people up against the migrants in Calais.
"What he should have been doing months ago is get the situation sorted out with the French. All of these people need to be assessed as to whether they're genuine refugees in which case they should be granted asylum, or they're not in which case they should be deported.
"But just sounding off against people describing them as a swarm is not the way to solve the problem for the all the lorry drivers and holiday makers who are having a nightmare on this side of the channel as well as on the French side."
And the Refugee Council, which works with refugees in the UK, said his comments were "irresponsible" and "dehumanising".
"This sort of rhetoric is extremely inflammatory and comes at a time when the Government should be focused on working with its European counterparts to respond calmly and compassionately to this dreadful humanitarian crisis," it added.
Mr Farage, meanwhile, suggested it was part of an effort by Mr Cameron to sound "tough" on immigration.
"I'm not seeking to use language like that," he told BBC Radio 4's Today programme. "The prime minister is trying to sound tough, whether he actually means it or not is a separate question."
But speaking about his personal experience of the chaos at Calais earlier that morning Mr Farage told ITV's Good Morning Britain: "A couple of times I've been stuck on the motorway and surrounded by swarms of potential migrants to Britain and once, even, they tried the back door of the car to see whether they could get in."
The UN Special Representative for International Migration accused British politicians of adopting a "xenophobic response" to the migrants crisis and said their the language had been "grossly excessive".
The SNP said the PM's language risked "inflaming what is already a tense situation", while Liberal Democrat leader Tim Farron said it risked "dehumanising some of the world's most desperate people".
Green Party deputy leader Shahrar Ali said: "People will be disgusted at Cameron's Calais comments and he should retract them immediately."
Downing Street responded to the criticism by saying: "The point he was making is that there are tens of thousands of people moving across Africa and trying to get to Europe."
David Cameron has previously warned politicians to choose their language "carefully" when talking about immigration.
In 2014 Defence Secretary Michael Fallon said he "misspoke" after saying that some areas of the UK could be "swamped" by immigrants without changes to European Union rules on movement.
The prime minister said at the time that Mr Fallon was "absolutely right" to have "corrected himself".
"It is right for politicians to raise concerns about immigration, but we should always choose our language carefully. [Mr Fallon] said this morning that he wished he had chosen his language in a different way, and I agree with that," he told the House of Commons.