This variant, first identified in India, is also less responsive to the Pfizer and AstraZeneca vaccines than the previous Alpha strain identified in Kent.
Early evidence showed the Pfizer vaccine offers 79 per cent protection against the Delta variant, compared to 92 per cent against the Alpha strain.
Researchers labelled the increased risk of hospitalisation “bad news”, but said the findings on vaccines were good news, as they are still very effective.
Under the timeframe previously set out by the Scottish Government, all of mainland Scotland was due to enter level zero restrictions on that date.
However, due to rising cases of the Delta variant, and the fact that just under half of adults have still not been fully vaccinated, this may be delayed.
The new research was published as national clinical director Jason Leitch warned that one dose of vaccine provides only 30 per cent protection against Covid-19.
The Delta variant has “changed the game”, he told BBC Good Morning Scotland on Monday.
He said: “Everything still works – distancing, ventilation, handwashing all still works – but what’s new about the Delta variant, and this is horrid, and we’ve learned it increasingly over the last few weeks, is the second dose is required for decent protection.
“You get about 30 per cent protection from one dose, you get 80 to 85 per cent from two … we vaccinated about half the country’s adults twice, now we need to get that up.”
Asked whether decision-makers should be confident that vaccines are effective enough against this variant to allow for a full easing of restrictions once more people have had two doses, the study authors said the vaccine protection is “substantive” and “strong”.
The peer-reviewed findings, from researchers at Edinburgh and Strathclyde universities and Public Health Scotland, are published in a research letter in the Lancet medical journal.
They are part of the EAVE II project, which studies the pandemic and vaccine rollout in real time, using anonymised patient data from almost the entire population of Scotland.
Researchers found the Delta variant had an 85 per cent increased risk of hospitalisation compared to the Alpha variant, after adjusting for age, underlying health conditions and other factors.
Hospital cases were measured as people admitted to hospital within 14 days of a positive PCR test, or who tested positive within two days of admission.
However, researchers noted the average length of hospital stay is shorter now that during previous waves of the virus.
Researchers studied the period from April 1 to June 6, during which there were 19,543 community cases and 377 hospitalisations where a specific variant was confirmed.
Of these, 7,723 cases and 134 hospitalisations were the Delta variant.
The majority of cases were found in younger people, but the authors linked this to more younger people being unvaccinated, and having more contact with the community, rather than to an increased effect of the Delta variant on that age group.
Preliminary evidence showed the Pfizer vaccine is slightly less effective when faced with the Delta variant, offering 79 per cent protection two weeks after the second dose, compared to 92 per cent in the case of the Alpha variant.
The AstraZeneca vaccine is also less effective – 60 per cent against the Delta variant and 73 per cent against the Alpha strain.
Researchers warned against any comparison between the Pfizer and AstraZeneca vaccines.
While the AstraZeneca jag appears to be less effective, it has been given to a different – and older – population in Scotland, so the results are not comparable, they said, adding the AstraZeneca jag may take longer to develop immunity.
The findings also showed the importance of people taking up both doses of vaccine, as the protection offered by a single dose was only about 30 per cent from the Pfizer vaccine and 18 per cent from the AstraZeneca vaccine, 28 days after the first dose.
Results also showed that protection is much lower in the month after the first dose, and the two weeks after the second.
Professor Aziz Sheikh, study lead and director of Edinburgh University’s Usher Institute, said the findings showed it was “absolutely fundamental” for people to take up both doses, and to wait the necessary two weeks after the second dose to be fully protected.
He said: “Over a matter of weeks the Delta variant has become the dominant strain of SARS-CoV-2 in Scotland.
“It is unfortunately associated with increased risk of hospitalisation from Covid-19.
“Whilst possibly not as effective as against other variants, two doses of the Pfizer-BioNTech and Oxford- AtraZeneca vaccines still offer substantial protection against the risk of infection and hospitalisation.
“It is therefore really important that, when offered second doses, people take these up both to protect themselves and to reduce household and community transmission.”
Dr Jim McMenamin, Covid-19 national incident director for Public Health Scotland, warned the results were preliminary and would need to be compared with further studies in other countries.
He also stressed the importance of people coming forward for vaccination.
“The bad news is we do appear to be able to show that the Delta variant does increase the risk of hospitalisation,” he said.
“However, what we are able to see from the information available to us is that our vaccines are still highly effective.
“Two doses provide strong protection against the risk of hospitalisation … or infection in the community.
“So that means we've got an opportunity to counter the threat of this Delta variant by encouraging uptake of both doses of the vaccine.
“That means if you have had your first dose, it’s an incredibly strong message here to come forward to get your second dose.
“If you have not yet come forward to be vaccinated, you need to come forward and take both doses of vaccine when they are offered to you.”
Professor Chris Robertson, of Strathclyde University, said it would be possible to get a fuller understanding of the situation when these preliminary findings were compared to results in other countries.
Some 761 new cases of Covid-19 were reported on Monday, representing 5.2 per cent of tests.
Just under 80 per cent of adults have been given a first dose of vaccine, while 55 per cent have had both doses.