It comes as Nicola Sturgeon continues to be under pressure from campaigners to bring forward legislation for buffer zones around abortion clinics, which would see protests banned within a certain distance of clinics.
In the letter to Mr Mason, who described such protests as “vigils”, whips criticise the MSP for a “lack of sensitivity”.
They wrote: “Your behaviour and conduct have been extremely disappointing, and we believe that you have brought the Parliamentary group into disrepute.”
“We would like to make it clear that we absolutely respect your right to hold your views on abortion and your right to freedom of speech and expression. We do not, however, believe that you have the right to impose these views on others.”
“The verbalisation of your views has caused great distress and trauma to many women and have also been regarded as misinformation by medical professionals.”
“We are dismayed that you have chosen to express your views in the media without approaching the group first to express the concerns you may have.”
“We consider this breach a serious one and are issuing you with a formal written warning.”
Mr Mason’s response, also reported by the Daily Record, claimed the pro-independence movement would be harmed if pro-life voices were “excluded”.
He also claimed his pro-life views have helped him in election campaigns in the past.
He said: “Traditionally the SNP has allowed freedom of conscience and a free vote on issues like abortion and assisted dying. I myself have spoken out on abortion at varying times and have always tried to be open about my views.
“I have had several messages from SNP members thanking me for speaking out as I have done. Is there not a risk that we lose SNP and independence voters if we are to exclude all pro-life voices in Parliament?”