The community testing programme, that initially opened to critical workers in February at the Guildhall in the city centre, has grown to include all workers who are not able to work from home.
It is thought around one in three people with coronavirus do not display symptoms, so it is hoped the testing scheme will reduce the spread in the city.
Councillor Matthew Winnington, the council's health and wellbeing boss, said: 'Testing, tracing and isolating limits transmission of Covid-19 and is vital in our fight to reduce infection rates and save lives.
'We know that one in three people who catch the virus don’t get symptoms, and so now we strongly encourage all workers who can’t work from home to book twice a week for free no-symptom tests. It's one of the most important contributions both employers and employees you can make right now keep our entire community safer.
'Tests take about 10 minutes, there is no need to wait for results on site as you are contacted as soon as the results are confirmed.'
Testing is still available for those who have received a Covid-19 vaccine and critical workers are advised to be tested twice a week.
Cllr Winnington added: 'Getting tested is voluntary, but I strongly urge all employers to support their eligible staff to take up this offer of free community testing.
'If they employ more than 50 people, businesses can register to set up their own workplace testing programme – while these are being set up, all those who can’t work from home are welcome to use the council's Guildhall test site.'
Appointments at the Guildhall centre are available from 8am to 8pm every day. Tests can be booked online at portsmouth.gov.uk/covid-testing.
Anyone with one or more coronavirus symptoms – a high temperature, a new, continuous cough, or a loss or change to sense of smell or taste – is advised to book a test at nhs.uk/coronavirus or by calling 119.
There are two symptomatic testing sites in Portsmouth which are open seven days a week at the Eldon car park in Somers Town and at North Harbour Road in Cosham.