The 10,000-patient Goodwood Court Medical Centre was visited by CQC inspectors on 9 June, and immediately closed.
Patients calling the practice now hear a message from NHS England advising them how to access medical care at a local walk-in centre.
The message says: 'It is with regret that we have to inform you that Goodwood Court Medical Centre is unable to provide clinical services with immediate effect from the 9 June 2015. This follows urgent action by the CQC to protect the safety and welfare of patients.
Patients sent to walk-in service
'NHS England is taking urgent action to guarantee your ongoing access to care and a letter will be sent to all affected patients about this immediately. If you need to see a GP or nurse immediately, you can attend Brighton Station Health Centre which provides walk-in services from 8am to 8pm, seven days a week.'
A CQC spokeswoman said this was only the 'second or third time' a practice has been immediately shut down after an inspection.
According to the NHS Choices website, based on 14 patients who completed Friends and Family tests 71% would recommend the Goodwood Court Medical Centre.
Practice manager Andrew Seymour has posted responses to some of a string of negative patient comments, however, pointing to problems with capacity while a branch surgery is undergoing refurbishment.
In one response, he wrote: 'The Eaton Centre was flooded in the summer and needs extensive refurbishment. This is currently taking longer than anyone anticipated, we are doing all that we can to move this process along and we will re-open as soon as possible. However until then all of our services are running from the Goodwood Court site, which has been something of a logistical challenge for the whole staff.'
Ruth Rankine, CQC deputy chief inspector of general practice, said: 'The CQC has recently inspected Goodwood Court Medical Centre, Cromwell Road, East Sussex. As a result of this inspection, on Tuesday 9 June, CQC is now taking action. We have been working closely with NHS England to ensure that patients continue to receive the service they are entitled to.
'Where providers are not meeting required standards, we have a range of enforcement powers we can use to protect the health, safety and welfare of people who use this service. When we propose to take enforcement action, our decision is open to challenge by the provider, and it wouldn't be appropriate for us to comment further at this stage. In due course we will publish a full report of our inspection, including details of any action we take.'