Probe in Norwegian killers UK links
Breivik, 32, who has admitted carrying out the Oslo bombing and Utoeya island youth camp shootings which killed at least 93 people, wrote a 1,500-page manifesto in English, published online and dated "London 2011".
Scotland Yard said a police officer had gone to Norway to help with the inquiry.
Foreign Secretary William Hague said the UK National Security Council (NSC) would meet today to discuss the attacks.
The document is reportedly littered with references to British politicians for allowing the spread of Islam. The Prince of Wales is criticised for his patronage of the Oxford Centre for Islamic Studies.
Former Prime Minister Gordon Brown is branded "a war criminal" and accused of aligning himself with Islamist terror groups.
Former British Prime Minister Tony Blair, Labour Party leader Jack Straw, Prince Charles and a number of national newspaper journalists are also mentioned.
In the document written in English, Breivik claimed he was recruited by two English extremists at a meeting in London in April 2002 attended by a total of eight people.
He signed the document with an anglicised version of his name – Andrew Berwick – and it was datelined "London, 2011".
The document said he used to have more than 600 members of the far-right English Defence League (EDL) as Facebook friends and had contact with EDL leaders.
However, the EDL has denied any official contact with him.
"We can categorically state that there has never been any official contact between him and the EDL," a statement on its website said.
"Our Facebook page had 100,000 supporters and receives tens of thousands of comments each day. And there is no evidence that Brievik was ever one of those 100,000 supporters," it said.
Speaking to BBC, Hague said the NSC would "look at the lessons to be learned" from the attacks in Norway.
"We have tight firearms controls in Britain and very tight controls on the sale of material needed to manufacture a bomb, and we have the most highly professionally trained firearms officers in the world, and we have the police and intelligence agencies working well together in making sure, best as we can, that the country is safe from attacks," he added.
Association of Chief Police Officers president Sir Hugh Orde told the same programme that the police and government had already run a "major exercise" to prepare for an attack similar to that which took place in Norway.