A suspected Pakistani bomber linked to an attack that killed 164 people is one of scores of terrorists who posed as refugees to enter Europe and are now an 'imminent' threat, investigators claim.

The Pakistani man, along with another, are believed to have arrived in Europe with two suicide bombers who blew themselves up outside the Stade de France in November.

It has also emerged that they were initially arrested in Greece after police realised their passports were among a batch stolen by Isis - but were released the continue their journey across Europe.

They were arrested in Austria after the Paris attacks, but it is feared they too had been plotting atrocities in Europe.

Muhammad Ghani Usman (left) and Adel Haddadi have been arrested at a refugee centre in Austria and are believed to be linked to terror groups and the suicide bombers who blew themselves up outside the Stade de France in Paris in November

Usman, linked to terror groups Lashkar-e-Taiba and Lashkar-e-Jhangvi, and Haddadi arrived on the Greek island of Leros (pictured) on October 3 on the same boat as two of the Paris suicide bombers

Muhammad Ghani Usman, who is believed to be a veteran bomb-maker for the Pakistani terror group behind the Mumbai bombings in 2008, has been detained near Salzburg, in Austria.

In a series of explosions, 164 were killed, along with nine of those responsible, and investigators believe he is a member of the group that claimed responsibility, Lashkar-e-Taiba.

Usman, 34, is being held on charges of participating in a terrorist organisation, along with Adel Haddadi, 28, a suspected Algerian Isis fighter linked to the terrorist gang behind the Paris attacks.

Investigators believe that both men are part of an Isis 'strike team' who have been sent to Europe to plot atrocities, posing as migrants to obtain entry, reports The Sunday Times.

A multinational investigation has also found that there may now be scores of terrorists who used the migrant flow to get into Europe.

A source close to it has warned that 'large-scale' attacks on European countries including Britain are now 'imminent'.

There is evidence that a specialist terrorist network based in Europe has been providing support to get more jihadists into the continent, including safe houses and fake documents.

Members of the gangs responsible for the November Paris attacks and those in Brussels in March were found to have used a network to get in and out of Europe, and to find safe haven when they arrived.

Usman, linked to Lashkar-e-Taiba and other group Lashkar-e-Jhangvi, and Haddadi arrived on the Greek island of Leros on October 3 on the same boat as two of the Paris suicide bombers.

A Greek police report says they all arrived on Syrian passports on a boat carrying 198 people, and Usman and Haddadi were travelling under the names Faycal Alaifan and Fozi Brahi.

The two men were arrested shortly after arriving at Leros (pictured) as police realised their Syrian passports were among 4,000 seized by Isis, but they were later released and allowed to carry on into Europe

The bombers used fake names Ahmad al-Mohammed and Mohammad al-Mahmod, and later blew themselves up at the Stade de France, part of co-ordinated attacks that killed 130 in France.

Usman and Haddadi were arrested soon after arriving in Greece as their passports were among 4,000 stolen by Isis and came up on a database accessed by Greek police.

However, they were released on October 28 and allowed to carry on with their journey across Europe, and applied for asylum at the Asfinag refugee shelter in Salzburg after the Paris attacks.

Phone records have linked the men to the suspected mastermind of the Paris attacks Abdelhamid Abaaoud (pictured)

But they were arrested again in December when a fingerprint search linked them to the passports stolen by Isis.

Investigators searched their phones and found that they had dialed numbers used by the Paris attackers, and the alleged ringleader of the attack, Abdelhamid Abaaoud.

This ringleader, who was killed after a shootout with police, had boasted to a friend that he had helped get 90 more jihadists into Europe as refugees, according to French investigators.

Two more asylum seekers at the Austrian centre were also arrested in connection with the search through the two suspects' phone.

Usman and Haddidi deny the charges but have been questioned by the French intelligence services and are expected to be extradited to France.

Investigators are trying to establish if they had intended to be involved in the Paris attacks but their arrests in Greece prevented them from doing so.

The international French investigation found that most terrorists in Europe obtained entry as refugees, travelling from Turkey to Greece on Syrian passports.

They then meet with Isis fixers who gave them fake Belgian ID numbers, and this allows them to travel freely thanks to the Schengen agreement, allowing Europeans to travel freely inside the EU.