The Parlemannews.ir website of the reformist bloc in parliament did not provide details of any reasons given for the travel ban.
Another source, who spoke on condition of anonymity, told AFP that the "police have informed Khatami that he may not leave the country."
By mid-afternoon, there had been no confirmation from official media of the order.
Khatami, whose 1997-2005 presidency saw a thaw with the West under his "dialogue of civilisations," had been invited to attend the annual nuclear disarmament conference in Hiroshima.
The former president has been the target of virulent criticism by regime hardliners since he backed former prime minister Mir Hossein Mousavi against incumbent Mahmoud Ahmadinejad in last June's presidential election.
Khatami, 66, has repeatedly denounced government crackdowns on the opposition, which denounced Ahmadinejad's re-election as fraudulent.
He has also called for the release of hundreds of people arrested during opposition demonstrations, some of whom have been sentenced to stiff prison terms.
