The House of Lords ruled last year that employees working abroad for UK-based organisations were still protected against unfair dismissal and discrimination under UK law.
This prompted a wave of litigation. About 40 separate cases are now being brought against UK-based charities.
The former Christian Aid employee claimed unfair dismissal and discrimination.
But the industrial tribunal ruled that, although the charity is based in the UK, overseas employees are not protected by British law if their main activity is abroad.
Maria Standingford, head of human resources for Christian Aid, said: "We're glad the tribunal has rejected these claims and we're grateful for the guidance it has given on the applicability of UK law to staff working overseas."