Home Secretary Alan Johnson has come to the defence of MI5 following the launch of a police investigation into allegations that some of its agents were complicit in torture.
Mr Johnson said he had "nothing but admiration" for the work of the security service and believed they operated "to the highest ethical and professional standards".
And he warned Britain's interests would be at risk if the service's counter-terrorism capabilities were "diminished and diluted".
Scotland Yard announced last week it was launching an investigation, at the request of Attorney General Baroness Scotland, into claims made by former Guantanamo Bay detainee Binyam Mohamed.
The 30-year-old Ethiopian national alleges MI5 knew he was tortured into confessing terrorist activities while in custody in Pakistan and Morocco and that MI5 officers fed questions to his interrogators through the CIA. The Government denies the claims.
Mr Johnson, who took on responsibility for MI5 when he was appointed Home Secretary in June, said: "I haven't sat around the last six weeks not looking into these things.
"I have looked very closely at them and I just say this: we have one of the best counter-terrorism capabilities in the world and we diminish and dilute it at our peril."
He added: "In my six weeks in this job I am so reassured and so amazed at the work that is going on, on our behalf, by people who do not have a voice, who are not able to express their views, who work in the most difficult and dangerous circumstances.
"I have nothing but admiration for them. As I am in effect their voice I will defend them and defend what they do, and it does worry me. I believe they work to the highest ethical and professional standards."
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His comments will be welcomed by the intelligence agencies amid concerns that public confidence in them has been shaken by allegations that they colluded with the torture of terror suspects.
