Pupils across Scotland have been experiencing problems accessing Microsoft Teams as the majority move to home learning.
A number of schools, pupils and parents have reported the technology running slowly or not at all.
It is one of the main platforms being used for remote learning with schools shut to most pupils until at least the beginning of February.
Microsoft Teams tweeted that the issue was being investigated.
A Microsoft spokesperson said: "Our engineers are working to resolve difficulties accessing Microsoft Teams that some customers are experiencing."
When pressed on whether demand as a result of home schooling was causing the issue, Microsoft declined to comment.
First Minister Nicola Sturgeon highlighted the problem during her daily coronavirus briefing.
"This is not an issue that is unique to Scotland or indeed unique to schools, but I understand Microsoft is currently working to address it," she said.
"More generally I don't underestimate how difficult this is both for young people learning away from friends… and for parents to juggle home schooling with working."
Ms Sturgeon was also asked about problems which were being experienced by users of digital learning platform Glow.
She replied: "It is not an issue with Glow. It is affecting Glow, but the core issue is not with Glow… the issue is with Microsoft Teams."
Two schools in Wishaw, North Lanarkshire, said the problem was a "national issue" although Renfrew High School urged pupils experiencing difficulties not to panic.
'Please be patient'
Clyde Valley High School tweeted: "Our online learning provision begins today for all of our pupils. Due to the very high demand for Microsoft Teams across Scotland, there may be issues initially getting logged on or accessing some files.
"This is a national issue on the site and may take a little time to rectify."
Coltness High School said: "Unfortunately it appears Microsoft Teams is struggling to cope with the traffic this morning.
"This is across Scotland and not isolated to Coltness. Pupils and staff are having difficulty loading files. We have reported the issue and hopefully this will be resolved soon."
Edinburgh City Council have texted all parents saying: "There is a city-wide problem with Microsoft Teams this morning. Please be patient as the council is working to resolve it."
It seems there is a wider problem with Microsoft Teams today due to the numbers using it. If you are trying and can’t get into a class please don’t panic. Hopefully Microsoft will resolve it soon.— RHS Digital Learning (@DigitalRhs) January 11, 2021
Aargh - IT probs! Learning @Home update - Microsoft is having issues across UK this morning in accessing Teams. This is affecting some of our Glow users. This is intermittent, so please try again later this morning or this afternoon. We'll let you know when it's fixed #SupportDG pic.twitter.com/81NkMrWAmG— D&G Council (@dgcouncil) January 11, 2021
A Scottish government spokesman said: "Microsoft has confirmed that this issue is affecting users in the UK and elsewhere in northern Europe. Education Scotland is working closely with the company to resolve the issues."
After one teacher complained to Microsoft Teams on Twitter, a staff member said: "We're currently investigating an issue where some users in the UK region are unable to access Microsoft Teams. We will provide further information as soon as this is available."
According to an Ofcom report in December, about 34,000 (1.2%) premises in Scotland were without a decent broadband connection, while superfast broadband coverage had increased to 94% of homes.
It also said that fixed and mobile networks in Scotland had "generally coped well" with increased demands during the pandemic.
It comes as plans for remote learning during the latest lockdown reveal big disparities between Scotland's 32 councils.
Not all pupils will be offered live lessons - instead the decision on the best approach has been left to individual schools and teachers.
Guidance on remote learning published by the Scottish government on Friday recommended a "a balance of live learning and independent activity".
The Scottish government said it had invested £25m to address digital exclusion in schools with funding allocations for digital devices and connectivity solutions made to all 32 local authorities.
More than 50,000 devices such as laptops have been distributed to children and young people to help with remote learning and the programme in total is expected to deliver about 70,000 devices for disadvantaged children and young people across Scotland.