UP to 5000 European nationals pose a potential terrorist threat to Europe, a senior law enforcement chief has told MPs, as a former head of MI5 warned that the West was facing circumstances similar to those that existed before the 9/11 terror attacks.

Rob Wainwright, director of Europol, told the House of Commons Home Affairs Committee his organisation has so far collected 2500 names of suspects, who had travelled overseas, from agencies across EU member states.

Asked how many so-called foreign fighters had left Europe, Mr Wainwright said yesterday (tue): "We're talking about 3000 to 5000 EU nationals.

"Clearly, we're dealing with a large body of mainly young men who have the potential to come back and have the potential or the intent and capability to carry out attacks we have seen in Paris in the last week," he said.

It is estimated that as many as 600 British nationals have travelled to Syria and Iraq.

Mr Wainright highlighted the need for greater co-operation with internet companies to tackle extremists, who were using networks much more aggressively than before.

"We have to have a closer, much more productive relationship between law enforcement and technology firms. Also, we need to have the right legalisation in place to allow the authorities to monitor suspected terrorist activity."

He sought to reassure MPs that Britain had "some of the most well-developed and effective counter-terrorism arrangements in the world" but that the terrorist threat was real.

During a Lords debate on the UK Government's Counter-Terrorism and Security Bill, Lord Evans of Weardale, who retired as director general of MI5 in 2013, urged the Government to tackle the "unfinished business" of giving the security services greater powers to access communications data.

In his maiden speech, the Crossbench peer said: "When I left MI5 in 2013, I felt cautiously optimistic we were over the worst as far as al Qaeda and Islamist terrorist attacks were concerned in this country. It seemed to me that we were making significant progress. Regrettably, subsequent events have proved that judgment to be wrong."

He said the current situation put him in mind of the al Qaeda training camps in Afghanistan before 9/11, which "drew would-be jihadists from across the globe".

"Those circumstances led to a series of attacks internationally and over a long period and I fear we may be facing the same situation...We are starting to see that."

Lord Wear also noted how inadequate security bred vulnerability and fear and "that in turn will tend to limit people's ability to contribute to civil society, will provoke vigilantism and will diminish people's ability to exercise the very civil liberties and human rights that we wish to sustain".

During the debate, part of the focus of which has been on the controversial call to give the security services more monitoring powers, Lord Bates for the Government claimed the UK was in the middle of a "generational struggle" against terrorism and that the security services needed the right laws to tackle the threat, the scale of which had been underlined by last week's massacre in Paris.

For Labour, Baroness Smith of Basildon said everyone agreed that the first duty of government was to protect the public but that counter-terrorism measures had to be "proportionate" to achieve a balance between freedom and security.

Meantime, Nigel Farage accused European authorities of showing "moral cowardice" in failing to tackle the problems associated with multiculturalism.

The Ukip leader claimed there were "no-go zones" for non-Muslims in "most big French cities" and there were problems across the continent.

The MEP argued that "big ghettos" had been allowed to develop and the authorities in the UK and elsewhere were turning a "blind eye" to issues including the sexual exploitation scandals in Rotherham and Rochdale, female genital mutilation and the application of sharia law.

The comments come after Mr Farage blamed a "fifth column" of Islamic extremists in Europe for terror atrocities including the Paris attacks and vowed to "be more courageous in standing up for our Judeo-Christian culture".

Communities Secretary Eric Pickles said: "It is disappointing that Nigel Farage is running our country down on American television, and his comments are both irresponsible and wrong.

"This Government is standing up for British values and British liberties. We've clamped down on the excessive use of foreign translation by councils, re-affirmed the importance of Christianity in public life, tackled divisive politics in Tower Hamlets, and are taking firm action to stop the criminal abuse of children.

"Mr Farage should show some leadership rather than pandering to people's worst fears."