The order for 76 Rover 45 cars was heavily influenced by the fact that the petrol versions of the model were available with an approved conversion to run on liquefied petroleum gas.
Other issues officials examined included the wholelife cost position of the range, along with carbon dioxide emissions and operating economy.
The initial order is made up of 1.8-litre petrol and 2.0-litre turbo-diesel models in hatchback and saloon format. There is also the possibility of adding more Rovers already fitted with LPG conversions.
MG Rover has given its entire dealership network the chance to become approved LPG installers through partnerships with EcoGas Systems and Landi Renzo. All conversions come with a three-year product warranty.
Tony Burkes, national fleet manager for HM Customs & Excise, said: 'Apart from the number of product merits for purchase, we were delighted to work with MG Rover Group, who provided us with very good customer service.
'The ability of our fleet to be fitted with approved LPG conversions produces a further opportunity to enhance wholelife operating costs.'
The retail price of equipping a Rover or MG with an LPG conversion is £2,195, but the cost can be offset by a TransportEnergy Powershift grant of 60% of the cost, and fuel is half the price of petrol or diesel.
John Granby, business development manager for MG Rover Group, said: 'We are developing our position as a major supplier in the corporate market, with large national fleets, government bodies and local businesses.'