Applying for Iranian visas can be a lengthy and complicated process. We recommend that you do not to purchase tickets for travel or make other travel arrangements before securing the visa.
Presently nationals of The UK, USA and Canada cannot travel to Iran without a guide. A company providing a guide will be able to apply for your "Authorisation Code" (equivalent to a letter of invitation) - this is issued by the Iranian Ministry of Immigration in Tehran. Once you have this, you have permission to apply for the visa. The authorisation code does not however, guarantee that you will be granted a visa.
You will then need to apply for the visa at the embassy of Iran in person in your country of residence.
Please note, the authorisation codes are consulate specific. That means you'll have to state at the time of application where you'll be getting the actual visa - this cannot be changed once the code has been applied for. If you decide to get the visa from a different embassy you'll need to get a new code.
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