Severn Trent is exiting Guyana after the government said it was failing to meet its targets for the supply of water to the South American country.
Severn Trent says it responded well to the emergency in 2005
Severn International's contract was due to end at the start of 2008 but is being terminated nearly one year early.
Severn said it was proud of its record in the country and had improved the quality of drinking water there.
Guyana suffered its worst-ever floods in 2005, severely damaging much of the country's infrastructure.
Harry Nawbatt, Guyana's water minister, said this was due to the firm's failure to meet targets, such as increasing the amount of money raised from water bills.
"We are terminating the contract with the company," he said. "The only reason is that they have failed to meet targets as we have together stipulated."
Severn Trent said the decision to terminate the agreement was "amicable", adding that it would continue to assist the government for two more months during a transition period.
It said it stood by its record of "solid improvements" over the past four years, including its efforts to provide emergency drinking water during the 2005 floods.
"Both Guyana Water and Severn Trent International would like to have achieved more but events such as the floods and delays in securing funds for capital investment have thwarted that," the firm said in a statement.
Severn took over the running of Guyana Water in 2002 after the state-owned firm was criticised for poor quality of service and financial management.
Severn said the country's water supply would now be run by Guyanese management.
Guyana is one of Latin America's poorest countries and its economy was badly damaged by 2005's floods. More than 40% of the population lost some or all of their possessions.
Severn Trent has been present in the Americas for more than 15 years, managing water services in Mexico and Trinidad & Tobago and providing consultancy services in Argentina, Peru and Guatemala.