Sir Richard Dalton, who is now a fellow at the Chatham House think tank, said he was dismayed that the former prime minister told the Chilcot inquiry he would consider taking military action against Iran.
During his evidence concerning the Iraq war Mr Blair, who is now a Middle East peace envoy, mentioned Iran 28 times and added: “I take a very hard line on Iran today and many of the same arguments apply.”
Mr Blair said Tehran’s actions made him even more afraid that a rogue state could supply weapons of mass destruction to terrorists than when he took Britain to war against Saddam Hussein in 2003.
But Sir Richard, who served as the UK’s ambassador to Iran from 2003 to 2006, said: “Iran is a completely different situation.
“We should be making it much clearer that the principal criteria for UK involvement in future wars in the Middle East is whether or not the safety and security of the United Kingdom and its territory is directly threatened.
“(Mr Blair’s argument) is based on the assumption that a nuclear capable state would hand those secrets to terrorists and that has never happened with nuclear capable states that have existed for many years.
“Moreover Iran has had chemical weapons technology and has not passed that on to terrorists, and to assume that there is such a danger from that is to misread Iran.”
Sir Richard also rejected Mr Blair’s claim that Iran’s collusion with al-Qeada had been responsible for destabilising Iraq after the invasion.