The Gun Carrier Mark I was a separate design, intended to carry a field gun or howitzer that could be fired from the vehicle. In service, it was mostly used for carrying supplies and ammunition. Forty-eight of them were built.
Initial production of the Mark I was to be by Fosters and Metropolitan: 25 from Fosters and 75 from Metropolitan, which had greater capacity in Wednesbury at the Old Park site of the Patent Shaft Company, a subsidiary of the Metropolitan. Metropolitan also received an order for a further 50 so that the Army would be able to raise six tank companies of 25 tanks each and set up further production under their Oldbury Wagon and Carriage Company. As there were not enough 6-pounder guns available for all 150 tanks, it was decided to equip half of them with just machine guns. A new sponson design with two Vickers machine guns in rotating shields was produced.