Prime Minister Brown has described the alleged plot as "very big" and said that investigators were looking at links with Pakistan.

In talks with the Pakistani leadership, Prime Minister Brown has raised concerns about links between terrorists in the UK and Pakistan and demanded "intensified efforts to crush militants."

He asked President Asif Ali Zardari to do more to root out terrorism emanating from Pakistan's territory after police in the UK arrested a dozen al-Qaeda suspects, including 11 Pakistanis, over a "very big terrorist plot".

"Prime Minister Brown telephoned President Zardari and the two leaders discussed matters relating to bilateral relations as well as the fight against terrorism," Presidential spokesman Farhatullah Babar said in Islamabad, without giving details.

Earlier, Brown said that there are "links between terrorists in Britain and terrorists in Pakistan. That is an important issue for us to follow through...."

"One of the lessons we have learnt is that Pakistan has to do more to root out terrorist elements in its country as well," Brown told Sky News.

Three of the four terrorists who carried out the London suicide bombings in July 2005 were young British men of Pakistani origin. They had travelled to Pakistan to receive religious and military training.

The issue of student visas represents a potential security nightmare for the UK. There are 330,000 foreign students in Britain and around 10,000 student visas are issued every year to Pakistanis alone.

Phil Woolas, the Immigration minister, has described the student visa system as "the major loophole in Britain's border controls".

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