The project is one of Africa's largest worth billions of dollars
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The British Foreign Office has referred a case of alleged corruption to the Serious Fraud Office for investigation.
The case involves UK company Mott MacDonald, which has been accused of bribing two Lesotho officials over a multi-billion-dollar water project.
The company is unaware of any reference to the Serious Fraud Office, and the Foreign Office has not contacted it.
The allegations emerged during the trial of two Basotho officials charged with receiving payments from companies.
Reatile Mochebelele and Letlafuoa Molapo were linked to a consortium led by a German company, Lahmeyer, which included Mott MacDonald.
Lahmeyer has been found guilty of corruption by a court in Lesotho and banned from further work paid for by the World Bank.
The Lesotho Highlands project involves building five massive dams as well as roads, bridges and electricity stations to supply water and power to South Africa.
The Lesotho chief executive of the scheme has already been sentenced to 15 years in jail.
The BBC's Martin Plaut says the tiny kingdom has been one of the few African states willing to prosecute not only its own officials but also Western companies that pay bribes.