London: The head of Britain’s foreign spy service admitted Thursday his organisation faced “dilemmas” to avoid using intelligence obtained through torture but insisted his agents never mistreated suspects.

In an unprecedented public speech, MI6 chief John Sawers also defended the cloak-and-dagger nature of the 101-year-old Secret Intelligence Service (SIS), whose existence was only officially acknowledged in 1992.

He said he read daily reports about people “bent on maiming and murdering people in this country,” many of them operating from the Afghan-Pakistan border but also from Somalia, Yemen and North Africa.

Britain’s security services have come under scrutiny in recent years over allegations of complicity in torture of detainees held following the September 11, 2001 attacks on the United States.

“Torture is illegal and abhorrent under any circumstances and we have nothing whatsoever to do with it,” Sawers said in the first ever televised address by an SIS chief, who is traditionally known as “C”.

However, he admitted that maintaining Britain’s principles was tough when working with more than 200 foreign partners who were not always “friendly democracies”.

“If we hold back and don’t pass that intelligence out of concern that a suspected terrorist may be badly treated, innocent lives may be lost that we could have saved,” he said. “These are not abstract questions... they are real constant operational dilemmas.”

In July, Prime Minister David Cameron set up an inquiry into the allegations of British complicity in torture, a move Sawers welcomed. But the spy chief expressed concern about information released when torture allegations were heard in court.

In February, a British court released secret evidence that a former British-based inmate of Guantanamo Bay, Binyam Mohammad, was subjected to “cruel, inhuman and degrading” treatment during questioning by US agents.

The information was made public in defiance of ministers’ warnings that such disclosures could harm relations with Washington. Sawers said that allowing countries who obtained information to control how it was used by other nations was “rule number one of intelligence sharing”.

“Because whenever intelligence is revealed, others try to hunt down the source. Agents can get identified, arrested, tortured and killed by the very organisations that are working against us,” he said.

“So if the control principle is not respected, the intelligence dries up.” The MI6 chief said that “nearly all” SIS agents were foreign nationals and insisted their work must remain secret, even if MI6 itself recruits publicly.

“Secrecy is not there as a cover-up. Secrecy plays a crucial part of keeping Britain safe and secure,” Sawers said, while noting that the agency was subject to significant government and parliamentary scrutiny.

One of its key roles was keeping tabs on militants overseas who might attack Britain, with more than one third of MI6’s resources directed towards international terrorism.

“The most draining aspect of my job is reading, every day, intelligence reports, describing the plotting of terrorists who are bent on maiming and murdering people in this country,” he said. “Few know about the terrorist attacks we help stop,” he said, adding: “Whatever the cause or causes of so-called terrorism, there is little prospect of it fading away soon.”

Sawers also made the case for intelligence as a tool against nuclear proliferation, saying that diplomatic pressure and sanctions were not enough to prevent Iran from developing a nuclear bomb.

“Stopping nuclear proliferation cannot be addressed purely by conventional diplomacy. We need intelligence-led operations to make it more difficult for countries like Iran to develop nuclear weapons,” he said. Tehran denies it is developing a nuclear bomb, insisting its programme is for purely civilian purposes.