Speaking at London's International Institute for Strategic Studies, Liam Fox said Britain's involvement in Afghanistan is a "national security imperative," the Times of London reported.
"A Conservative government would be sympathetic to a request for an increase in the number of British troops to help expedite the training of Afghan security forces," Fox said.
Elections must be held before June 2010 and Prime Minister Gordon Brown's Labor Party faces a tough challenge from the Tories.
The Times report said the current British government rejected a request this year for an increase of 2,000 to 2,500 British troops in Afghanistan. Fox warned a failure in Afghanistan would be a "shot in the arm" for jihadists around the world, the report said.
"Be under no illusions -- it would fuel latent fundamentalist sentiment within the U.K. and other European countries," he said.