The Delta variant, first identified in India, is now the most common strain of new Covid-19 cases in Wales.
Case numbers involving the variant have risen by more than 70% in four days since 10 June - to 315 - according to Public Health Wales (PHW).
It is now the dominant strain in Wales and PHW expects case numbers to rise further over the coming weeks.
In all, there have been 247 new Covid-19 cases reported in Wales, but no new deaths, latest figures show.
Wales' overall seven-day case rate has slowly risen to 18.4 per 100,000 - from 9.4 this time last week - and is the highest since 5 April.
Clusters of the Delta variant have been identified in Conwy, Rhyl and Denbighshire, which are now showing the highest case rates in Wales.
However, most of the health boards in Wales have now reported cases, which are not being connected to international travel, PHW said.
Incident director Dr Giri Shankar said he was "concerned" at the rise that was being fuelled by increased mixing, following an easing of restrictions.
"This is a significant development, but one we have predicted, as we know the Delta variant is easier to catch than the previously dominant Alpha variant," he said.
"Latest evidence shows that both Pfizer and AstraZeneca vaccines are effective against the Delta variant after two doses and so it's so important to take up the offer of both vaccines."
Figures are often higher on a Monday as those released cover a 48-hour period up to 09:00 on Sunday, with data no longer published on Saturdays.
The number of people who have died in Wales with the virus remains at 5,572 though total cases has hit 214,004.
But Covid hospital numbers continue to remain low, with only two patients in critical care across Wales on Friday, out of 18 patients with confirmed Covid in hospital beds.
Where are the new cases?
Every local authority in Wales has reported new cases, though north Wales has seen the largest numbers.
Of the newly reported cases, 28 were in both Conwy and Flintshire, 21 in Denbighshire and 20 in Wrexham.
Cardiff also reported 20 new cases.
What about the Covid case rate?
The weekly case rate per 100,000 people remains highest in Conwy county, where a cluster of cases of the Delta variant has been confirmed, having risen again to 46.9.
Denbighshire's rate is now 39.7, with a cluster in Rhyl. Swansea, with a cluster of cases linked to Central Swansea, has also surpassed 30 per 100,000.
But Merthyr Tydfil, which before Christmas had the UK's highest case rate, now has the lowest in the mainland UK on five cases per 100,000. Caerphilly, Powys and Monmouthshire are all in the lowest 10 in the UK.
The Wales case rate has risen to 18.4 cases per 100,000, its highest point since 5 April and double what it was a week ago.
But it remains well below other UK nations and every English region.
How many people have been vaccinated?
A further 2,981 people have received a first vaccination and the total now stands at 2,216,031 - equivalent to 70% of the total population.
Another 6,663 people have now had their second vaccination, so that total now stands at 1,384,673, which represents 44% of the overall population.
Find out how the pandemic has affected your area and how it compares with the national average:
The postcode search has been updated to replace data for health boards in Scotland with data for local councils. In England, data for county councils has been replaced with data for district councils. Figures for boroughs and unitary authorities remain unchanged.