Bulgarian and Romanian migrants will not be stopped from coming to the UK by the end of 2013, Home Secretary Theresa May said.

Both countries joined the EU in 2007 but strict controls were put in place preventing their residents moving to the UK and other member states. These expire at the end of December 2013.

She said ministers could only now look at what attracted migrants to Britain, such as the NHS and benefits system.

But the Home Secretary said the government was still aiming to reduce immigration to the tens of thousands, a key pledge of the Tories' 2010 election manifesto.

Speaking to Andrew Marr on the BBC, Mrs May said: "I am looking at free movement generally across the EU. Originally it was free movement of workers, it has been extended over the years.

"First of all, there is a growing group of countries in the European Union who are very concerned about the abuse of free movement. That is looking particularly at issues like sham marriages, forged documents and so forth, and we are working together to reduce the abuse of free movement.

"I will be looking at transitional controls on Romania and Bulgaria at the end of December 2013. I will be looking at what we call the pull factors. What is it that attracts people sometimes to come over here to the United Kingdom? Looking at issues such as benefits and access to the health service and things like that.

"There are no further transitional controls that we can put on. Transitional controls end in December 2013 but that is where the importance of looking at what it is that is attracting people to come here, in terms of things like our benefits system, and access to the National Health Service, are so important."