Scotland's health secretary says supplies of the Pfizer Covid-19 vaccine will be "particularly tight" in the weeks ahead.
That brand is being used for most of the jabs being given out to younger age groups, based on advice by the Joint Committee on Vaccination and Immunisation.
But Humza Yousaf is concerned it will slow the vaccination programme down.
He has written to UK health secretary Matt Hancock, asking what more can be done to address the situation.
His letter said: "The tireless work of all those involved in the vaccination programme means we can continue to stay ahead in the race against the virus.
"I commend the work of the UK Vaccines Taskforce in securing vaccine supply to support this effort and I value the strong working relationships that our officials have developed over the course of the last year on this.
"In Scotland, similar to our four nations counterparts, we are working hard towards the end of July target for offering first does to all those aged 18 and over.
"This has become even more important as we implement the adjustments to dose schedule set out by JCVI and deploying additional accessible routes to reach communities given the increase in prevalence of the Delta variant.
"I am very clear that this is a race and we must therefore do absolutely everything in our power to accelerate our efforts to vaccinate as many people as possible.
"I appreciate we are all doing what we can in this regard, but I am conscious that the recent changes by JCVI on the role of AstraZeneca, have placed increased pressure on our supply of Pfizer in particular.
"The latest forecast on supply would indicate that Pfizer will be particularly tight over the next few weeks and as a consequence the programme won’t be able to accelerate as much as I would wish.
"I know that the Vaccines Taskforce and colleagues in BEIS are working incredibly hard to maximise volumes of all supply including Pfizer.
"Notwithstanding this, I would be grateful to discuss what more could be done to further increase the available supply in the coming weeks to support efforts to deliver the programme at pace.
"I would of course be happy to discuss this at our next four nations ministerial call if that would be useful."
The Scottish Government said the letter was written ahead of discussions between the four nations today.
We have asked the UK Government for a response.