A former senior intelligence official has accused Tony Blair of attempting to "rewrite history" over what he really knew about President Bashar al-Assad's chemical weapons capability.

Last week Blair launched a passionate defence of his decision to invade Iraq, during which he claimed that the west did not know Assad was manufacturing chemical weapons until the regime began using them in 2012.

However, John Morrison, former head of the defence intelligence staff and the deputy chief of defence intelligence between 1994 and 1999, says in a letter to the Observer that, for more than a decade before the 2003 invasion of Iraq, Blair must have been aware that Syria had a chemical attack capability because intelligence officials had told him so.

Morrison, also an ex-member of the joint intelligence committee (JIC), added that Syria's chemical stockpile was a recurring theme in reports by the JIC, which advises the prime minister and the cabinet of security matters. "The issue was not whether he [Assad] had them but when and how he might use them," he writes.

Last Wednesday experts from the world's chemical weapons watchdog revealed that they believed toxic chemicals such as chlorine are being used in a "systematic manner" in Syria after investigating several incidents in the north of the country.

Morrison also revisits the controversy over Iraq's possession of weapons of mass destruction, saying that, since the 1991 Gulf war, British intelligence officials had assessed Saddam Hussein as having a "breakout capability" to reactivate his WMD programme, but that this was nothing to do with the reports by UN weapons inspectors in the runup to the invasion 11 years ago.

Morrison writes: "One wonders whether Blair read the assessments we provided him, is trying to rewrite history to his benefit, or is suffering from some prime ministerial false memory syndrome. Whatever, he should not be allowed to get away with untruths."