This comes as the number of coronavirus cases in the popular holiday destination rise.
“#Portugal will be added due to instability on case numbers,” Charles tweeted.
Portugal was only added to the travel corridor list two weeks ago.
A seven-day rate of 20 COVID-19 cases per 100,000 is understood to be a key threshold for whether a country is removed from the FCO ‘safe’ list.
Unfortunately, although Portugal’s rate previously dropped to 14.9 it has now busted the threshold.
It currently has an average of 22.5 cases per 100,000 people.
If Portugal is added to the ‘red’ list, travellers will likely have until 4am on Saturday, September 5 to return.
After that time they will have to quarantine for 14 days on arrival back in the UK.
Italy and Greece are two other major holiday destinations jet-setters have their eye on – so how safe are they?
The good news is that, according to Charles, neither will lose their ‘green’ status this week.
“In the weekly @transportgovuk #quarantine review my understanding is that #Greece #Italy will not be added and stay as open corridors,” the expert tweeted.
This comes after Scotland announced anyone returning from Greece to Scotland would have to self-isolate on return.
Wales has applied the same rule but only from the Greek island of Zante.
Greece currently has a coronavirus rate of 14.4 and Italy 14.7.
The government announcement is expected this afternoon.