The fellowship said its 30-second film aimed to highlight the cost of war in money and lives.
But it said the advert had been rejected for broadcast on live television because it was a matter of "public controversy".
'Political' advertising-advertisers cannot advertise on television,”
Clearcast spokesperson
Mererid Hopwood, a member of the Fellowship's executive committee, said: "If peace is a matter of 'public controversy,' then war must belong to the same category.
"And yet, the armed forces have been allowed to advertise regularly on S4C."
