There have been delays in sending out letters to "extremely vulnerable" patients urging them to self-isolate for at least 12 weeks.
Earlier this month it was announced that people most at risk of suffering serious complications from Covid-19 should adhere to the strictest isolation guidelines to "shield" them from the virus.
That group includes some cancer patients, organ transplant recipients, people with severe lung conditions and weakened immune systems, and pregnant women with heart conditions.
These people were told to stay at home at all times for at least 12 weeks and aim to remain two metres (6ft) away from anyone they live with.
But the British Medical Association (BMA) in Wales is reporting that many people in this category have still not received a letter with the self-isolation instructions.
Dr Phil White, BMA Cymru Wales GP committee chairman, said these letters must be sent as a "matter of urgency" so worried patients don't turn to GPs in their droves for advice.
He said: "The NHS Wales website is rather unhelpfully advising people who have not yet received a letter to contact their GP for advice.
"GPs have not been provided with a list of patients in their area that will be receiving a letter and we are concerned that the phone lines will become blocked by the sheer number of patients seeking advice – not knowing where else to turn – and when they do get through we won’t be in a position to help them because we have been given no information.
"At what is an unprecedented time for the health service as a whole this sort of administrative nonsense will cause unnecessary stress to patients and simply add pressure to an already overstretched service.
"NHS Wales must send these letters as a matter of urgency to provide patients with the information they need at this difficult time.”
In response a Welsh Government spokesman said: "All those identified as being at a very high risk of severe illness from coronavirus (due to a serious underlying health issue) will receive a letter from the Chief Medical Officer for Wales by no later than the end of this week.
"Letters were issued last week to GPs explaining this process along with a copy of the letter issued to patients. We engaged with the BMA and Royal College of GPs on this.
"As planned and to ensure all high-risk patients are contacted, GPs were today provided with a list of those who've been sent letters to check against their known patients.
"They will then contact any additional high-risk patients not identified to provide the advice outlined by the Chief Medical Officer. Those patients who are waiting for letters should not contact their GP surgery before the end of this week.”
