The Southampton midfielder is joined by Ryan Christie and Kieran Tierney who, along with a physiotherapist and a masseur, will have to self-isolate for 14 days.
Armstrong originally tested negative on arrival at Scotland’s training base at the Oriam on Monday but later returned a positive test from Uefa the following day. He will self-isolate for 10 days.
All other members of the squad have returned two rounds of negative results, however Arsenal full-back Tierney and Celtic midfielder Christie will have to sit out regardless, having been in close contact with Armstrong.
All three will miss the forthcoming international triple-header, which consists of the Euro 2020 play-off semi-final against Israel at Hampden and the Nations League matches against Slovakia and the Czech Republic.
Christie will also be absent for Celtic’s clash with Rangers in the first Old Firm game of the season on October 17. The Parkhead side already could be without Odsonne Edouard after the striker tested positive for Covid-19 while on international duty with France’s under-21s.
This is a significant blow for Steve Clarke as he’ll be without three players who featured in the 1-1 Nations League draw with Israel last month. The national team boss will also be without the services of Liam Palmer, Scott McKenna and Oliver Burke, who have dropped out the squad due to injury.
Scotland are trying to qualify for their first major tournament since the World Cup in France in 1998.
Clarke said: “While this is obviously disappointing news the most important thing is the health and safety of the individuals involved and the wider group.
“We have informed the respective clubs from whom we have borrowed the players and backroom staff and we now have to prepare for a huge match ahead tomorrow.”
SFA chief executive Ian Maxwell added: “We were made aware of the positive test this morning and I reiterate Steve’s view that the health and safety of those involved is paramount. We will maintain constant dialogue with Dr John MacLean to ensure that those who are required to self-isolate are given the full support of the Scottish FA in conjunction with their clubs.”