Lengthy queues had already formed outside the store in Commercial Road by 7.15am today.
But the line has continued to grow and now stretch back down to the Job Centre in Portsmouth, with hundreds of shoppers now queuing up.
The high street retailer is open from 8am to 8pm every day this week.
The queues outside Primark are reminiscent of the huge lines seen in Commercial Road in June 2020, when shops reopened following the first lockdown.
Non-essential retail as well as salons, barbers, pubs and restaurants are allowed to reopen today in Portsmouth for the first time since before Christmas.
It is the latest stage in the easing of lockdown.
The reopening of businesses comes after pupils returned to schools at the start of March and then outdoor socialising was allowed again a few weeks ago.
We will be bringing you all the latest updates as shops reopen across the Portsmouth area this morning.
You can follow along in our live blog – here.