The Government is investigating claims by the man believed responsible for killing 76 people in Norway that he had links with far-right groups in England, Prime Minister David Cameron has confirmed.
Mr Cameron said the comments made by Anders Behring Breivik were being taken "extremely seriously", before adding that Britain would review its own security in the wake of the double massacre.
Accused Breivik, 32, has confessed to being behind the Oslo bombing and the gun rampage at the Utoya Island Labour Party youth camp that left eight and 68 people dead respectively. However, he denies criminal responsibility and on Monday entered a not guilty plea at a closed hearing in an Oslo court - despite telling investigators he never expects to be released from jail.
Judge Kim Heger, who remanded Breivik in solitary confinement for eight weeks, said he had told him that he wanted to save Europe from a Muslim takeover and claimed that two further cells existed in his organisation.
He said Friday's bombing and shooting rampage was intended to send a "strong signal to the people" and deter future recruitment to the Labour Party, which he blamed for allowing "mass imports of Muslims", said the judge.
In a rambling 1,500-page manifesto posted online shortly before the attacks, Breivik stated that he was acting alone but had been recruited to the radical cause by two English right-wing extremists at a meeting in the UK in 2002.
Mr Cameron, who on Monday discussed the massacre with security chiefs and senior ministers at a meeting of the National Security Council, said: "We are still investigating these claims, so I don't want to give out partial information. We want to get to the bottom of this before making public announcements. But we take these things extremely seriously."
Foreign Secretary William Hague confirmed that a UK police officer is helping Norwegian investigators who are looking into Breivik's possible contacts.
The English Defence League issued a statement insisting that it has never had "any official contact" with Breivik and that there was no evidence that he ever registered as a supporter on the EDL Facebook page.
On Monday, Norway's prime minister Jens Stoltenberg and King Harald led the nation in a minute's silence in memory of those killed. Around 150,000 people also took to the streets of Oslo while other torchlit vigils took place in other towns and cities.