Appointments will be available fro this month and the booking must be at least 24 weeks after the second dose.
Additionally, 16 and 17-year-olds can use the portal from December to book a slot for their second dose of the vaccine as long as 12 weeks have passed since the first dose.
However, anyone with a confirmed case of the virus since the first dose is being asked to wait 12 weeks after that.
The portal is already open for the 50-59 age group, unpaid carers who are 16 and over, and those aged 16 and over who are household contacts of immunosuppressed individuals. As a bonus, anyone in earlier groups who missed an appointment can also book online.
As the portal is now being opened to even more cohorts, 50-59-year-olds who have not yet booked an appointment for their booster will be contacted shortly to invite them to make a booking as soon as possible via the portal or national phoneline.
Dr Jane Burns, Medical Director, NHS Lanarkshire, said: “The expansion of the COVID-19 vaccine booster to those aged 40-49 marks another significant milestone in the Autumn/Winter vaccination programme.
“Boosters are just as important as the initial vaccinations. Six months after the second dose, immunity levels wane with increasing risk of further infection. The booster dose improves your level of protection significantly and is the best way to protect your health and those around you.
Health Secretary Humza Yousaf said: “We aim to vaccinate people as quickly as possible in line with clinical guidance and over the course of the Autumn/Winter vaccination programme, we will see up to 7.5 million flu and COVID-19 booster vaccinations administer
To use the online portal, individuals will need their unique vaccination username which can be found on any previous appointment letter. If this is not readily available, it can be located or reset at NHS Inform’s “recover username” page.
Those without online access, can book an appointment through the National Helpline 0800 030 8013