Illegal immigrants are trying to "break in" to the UK, David Cameron has claimed as he defended his description of the flow of people across the Mediterranean as a "swarm".

The Prime Minister said many of those attempting to get into the UK were doing so for economic reasons and he was determined to make sure the border was secure.

But he insisted that the UK had been "one of the most generous countries in Europe" for granting asylum to refugees fleeing persecution or danger.

Asked whether the UK was doing enough to take in people seeking asylum he told BBC Radio 4's Today programme: "I think we do. If you take a 25-year view, Britain has always been one of the most generous countries in Europe for giving people asylum.

"But what we can't do is allow people to break into our country. A lot of people coming to Europe are coming in search of a better life, they are economic migrants and they want to enter Britain illegally and the British people and I want to make sure our borders are secure and you can't break into Britain without permission."

The chaotic scenes at the Channel crossings, with migrants attempting to stow away on vehicles travelling on ferries or through the tunnel, have been one of the biggest issues faced by Mr Cameron during his first 100 days of majority Tory rule.

The Prime Minister caused controversy when he blamed the crisis on a "swarm" of migrants risking the dangerous crossing of the Mediterranean to reach Europe.

"I was explaining that there are a large number of people crossing the Mediterranean and coming from the Middle East and coming to Europe," he said as he defended the use of the word.

"I was not intending to dehumanise."

He added: "I don't think it does dehumanise people. Look at what Britain's response has been. I made sure that we sent the Royal Navy flagship to the Mediterranean which has rescued thousands of people, saved thousands of lives."

The UK's aid budget had been spent on helping to stabilise some of the countries which had been the source of migrants, he added.

The Prime Minister defended his measures to restrict legal migration to the UK, including his proposals for welfare restrictions for EU citizens coming to the UK.

A four-year ban on access to tax credits and child benefit for new migrants is part of the UK's aim to stem EU migration to the country, but the BBC has reported that legal advice suggests the restriction will have to be extended to British workers to prevent "direct discrimination", effectively preventing them from claiming until they are 22 and have been in work for the required period.

Mr Cameron said: "There's legal advice that floats around and people make this argument and that argument. My point is we have only just started the renegotiation process to put in place the welfare changes that I set out in the manifesto which I remain committed to."

He added: "I set out what I want to achieve in our manifesto ... I want what is in our manifesto, which is that you have to work here for four years before you are entitled to benefits.

"That's what we set out in the manifesto. We have only just started the process of going through the technical discussions about what we can do in terms of this negotiation.

"What's good is that it's under way, so the process has started."

The Prime Minister said the in-work benefits acted as a draw for workers from the rest of the EU.

"I'm not saying people all come from Europe to come and claim unemployment benefit in Britain, it's not that that's happening so much as our in-work benefits are incredibly generous and act as an additional draw because you can get something like £8,000 extra on top of your salary coming from Europe to work in Britain.

"We need to reduce that draw to make our welfare system work better for us and to act as less of a draw for migrants coming in from Europe.

"We have had these circumstances in the last few years, we have created more jobs in Britain than the rest of the EU put together, so it's not surprising people have wanted to come."

Shadow home secretary Yvette Cooper said Mr Cameron should have apologised for calling migrants a "swarm" as she repeated her call for the UN's refugee agency to intervene in Calais.

She said: "Cameron should apologise for his divisive language not defend it, instead of rhetoric we need a practical plan.

"You don't call people a 'swarm' - they are fathers, mothers, daughters, sons. Instead we need a humane and practical plan, starting with getting the UNHCR to assess people camping at Calais to make sure refugees get support, illegal travelling is prevented, and tackle the problems on the coast."