Archive footage of the radical Islamist cleric Abu Hamza, who will be extradited to the US after a ruling by the European court of human rights. Reuters and ITN

The Queen lobbied the then home secretary to secure the arrest of Abu Hamza al-Masri, the radical Islamist cleric who faces imminent extradition to the US, the BBC's security correspondent has said.

Frank Gardner said the monarch personally told him she was aghast that Abu Hamza could not be arrested during the period when he regularly aired vehemently anti-British views as imam of Finsbury Park mosque in north London.

During an interview with BBC Radio 4's Today programme about the wider issue of the 54-year-old's newly approved extradition to the US, Gardner said of Abu Hamza's former activities that there was a sense MI5 had been too slow to realise how dangerous he was in radicalising other people.

Gardner continued: "Actually, I can tell you that the Queen was pretty upset that there was no way to arrest him. She couldn't understand – surely there had been some law that he had broken? In the end, sure enough, there was. He was eventually convicted and sentenced for seven years for soliciting murder and racial hatred."

A clearly surprised James Naughtie, interviewing Gardner, described this revelation as "a corker". Gardner said: "Yes, I thought I'd drop that in. She told me."

Gardner said: "She spoke to the home secretary at the time and said, surely this man must have broken some laws. Why is he still at large? He was conducting these radical activities and he called Britain a toilet. He was incredibly anti-British and yet he was sucking up money from this country for a long time. He was a huge embarrassment to Muslims, who condemned him."

The general convention for people conversing with the Queen, for example at palace receptions, is that whatever is said remains off the record.

A Buckingham Palace spokeswoman said she had no comment on Gardner's interview.

Gardner did not specify which home secretary was lobbied, but it appears most likely to be David Blunkett, who held the post from 2001 to 2004, at the peak of Abu Hamza's infamy before he was arrested. Following his initial arrest in August 2004, Abu Hamza was convicted in 2006 of 11 charges connected to soliciting murder and inciting racial hatred.

There was no immediate comment from Blunkett's office.

A spokesman for the republican pressure group Republic said the comments, if true, showed the monarch had needlessly "waded into the debate". He said: "It is up to parliament and the courts to deal with these complex issues, not the Queen. Monarchists argue the Queen always remains above politics. Clearly that is not the case."

The government has battled for eight years to secure Hamza's extradition to the US, where he is wanted in connection with alleged plans to establish a terrorist training camp in Oregon, as well as claims he provided material support to the Taliban. He is also wanted in connection with allegations that he was involved in hostage-taking in Yemen in 1998.

Abu Hamza's fight against extradition ended on Monday when the European court of human rights rejected his appeal, as well as those of four other terrorism suspects, and agreed an earlier ruling that their human rights would not be violated by the prospect of life sentences and solitary confinement in a US prison.