But analysts said Sawer’s comments pointed to an assumption that Tehran was already developing a nuclear weapon.
"Most Western intelligence agencies believe that Iran has not made the decision to acquire a nuclear weapon, but is amassing the capability to weaponise when it decides to do so," analyst Dina Esfandiary of the London-based International Institute for Strategic Studies told AFP.
"He (Sawers) seems to be completely glossing over that," she said.
"At the moment, Iran has enough low-enriched uranium for four or five bombs if further enriched," she said, explaining that only highly-enriched uranium can be used for weapons.
Patricia Lewis, an international security expert at London’s Chatham House think-tank, said she was surprised by the claims that MI6 had stopped Iran getting nuclear weapons by 2008.
"I am surprised by the date because, if you look at IAEA (International Atomic Energy Agency) documents, the general understanding is that any weapons activities that Iran had stopped in 2003," she told AFP.