Mr Pratt and Mr Wallace were arrested on 31 March on the Yugoslav border. They were intending to cross into Croatia and drive south towards Montenegro to help the Kosovo refugees there. The border guards were suspicious of the men's laptop computers, files and mobile telephones.
On 12 April, the Yugoslav authorities displayed Mr Pratt on television in Belgrade, confessing to collecting information about Kosovo and the effects of the Nato bombing campaign.
Belgrade has rejected appeals for the men's release, including one from the UN secretary-general, Kofi Annan.