I am Pakistani citizen and resident of UK. I am travelling from London to Pakistan through aeroporti di Roma-Fiumicino(Italy).My question is while travelling from T1 to T3 of airport do I have to apply for a transit visa?
If as the title of the question implies, this is a self-transfer, which means that the two flights are on separate tickets, then you most likely need a full visa.
Since the tickets are separate, the first airline considers that Italy is your final destination, and they want to be sure you can enter that country. They don’t care you are only staying there for a few hours, this is not their concern: they sold you a flight from A to B, you need to be able to enter B.
If you have any checked bags, this is 100% certain as you will need to enter Italy to retrieve your bags and go to check-in/bag drop.
Even if you don’t have any checked bags, it’s extremely likely. Some airlines or individual agents may accept that you will stay airside and accept just an airport transit visa, but it is extremely difficult to be sure they will, so it’s a very risky proposition.