Cornish MP Colin Breed has criticised a decision to dispose of Plymouth's waste in a landfill site in the county once the site in Chelson Meadow is full.
The methane from the waste will be converted into electricity
The city council has signed a seven year contract with Viridor Waste Management to take the rubbish to Lean Quarry near Liskeard from 2008.
The contract will see lorries taking 24 trips a day along the A38.
Viridor's managing director said the waste would be used to generate methane which could then be used as power.
Mike Hellings said the site in Liskeard could cope with the extra rubbish as it holds three million tonnes of waste.
Mr Breed, Liberal Democrat MP for South East Cornwall, had said it was "irresponsible" of Plymouth to dump its waste in its neighbour's back garden.
Devon County Council wants to expand the landfill site in Deepmoor near Torrington, which is also expected to reach its capacity within the next four years.
The county's other two landfill sites, at Broadpath, near Uffculme, and Heathfield, near Bovey Tracey, have space for the next 10 years.
Councillor Chris Pattison said: "This contract will give Plymouth medium-term certainty as well as flexibility to develop its long-term strategy for the city's waste, considering the latest technologies."
The council said the contract was an interim measure to allow it to investigate, plan and install new systems.