The Prime Minister turned down the demand from Labour's Denis MacShane just 24 hours after the House of Commons switched to brands of coffee which are fairly traded for its restaurants and cafes at Westminster.

"Individual Government Departments are responsible for taking decisions on the goods and services they acquire, taking into account the Government's policy of seeking value for money for the benefit of the taxpayer," he said in a Commons written reply.

"However, as the Secretary of State for International Development has made clear, the Government is also committed to promoting and supporting ethical trading wherever possible."

The House of Commons made the change after a campaign by MPs who wanted to make sure that the 2,500 cups drunk every day at Westminster have nothing to do with exploitation of Third World farmers.