Police have issued a closure notice on The Rockstone pub in Bevois Valley after they say it failed two under-age test purchase operations.
Alcohol was sold to 16-year-olds during the operations.
The pub will now not be allowed to sell alcohol between 12pm on Tuesday, August 2 and 12pm on Sunday, August 7.
The pub's owners reportedly said in a statement that a member of staff had been dismissed following the second check, and that the pub operates a strict Challenge 25 policy, with members of staff requesting proof of identity from under 25s. It said these were only occasions the "family-friendly gastropub" had failed spot checks since opening in 2011.