Mr Weir had previously lodged an Early Day Motion in Parliament opposing the whole principle of zonal pricing which received widespread cross party support.
Mr Weir said: "Coming just after the Royal Mail announced that the universal service obl
igation led to a loss in the last financial year, and in the midst of the independent review of postal services, this is an extremely worrying development.
"Although the original application only referred to business mail, this could potentially lead to an attack on the whole principle of the universal service obligation (USO) and universal tariff.
"A huge number of small and medium sized enterprises, especially in rural areas, rely on the Royal Mail, a fact accepted by the independent review.
"If there was to be any threat to the USO it could have a devastating effect on the rural economy.
"Any attempt by the Post Office to tamper with the principle in any way must be resolutely resisted."
Mr Weir, however, welcomed the declaration by Postcomm that, contrary to earlier reports, they would not support the axing of Saturday deliveries.
Mr Weir added: "I welcome the unequivocal statement by Postcomm that they continue to support the six-day delivery. That again is vital to business in rural Scotland."
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