Britain and France emphasised the force would be entirely defensive and only deployed once lasting peace in the region was agreed.
The United States and Iran, as warring parties, did not attend the talks.
The two-week ceasefire between the US and Iran was due to expire today, but US President Donald Trump said shortly before the deadline that he was extending it to give more time for negotiations.
Both sides have accused the other of breaches, and Trump said he was maintaining a US blockade of Iranian ports.
Downing Street had announced before the Paris meeting that there would be a military planning summit this week without giving details.
The conference will take place at the Permanent Joint Headquarters in northwest London, the UK’s command centre for overseeing and managing joint military operations.
-Agence France-Presse