"This additional UK funding will allow the Special Tribunal for Lebanon to continue its important work to end impunity for political violence in Lebanon," Fletcher was quoted as saying by Lebanon's The Daily Star.
According to the newspaper, the ambassador announced the contribution during a phone conversation with Lebanese Prime Minister Tammam Salam.
Fletcher said he had discussed with Salam the importance of the rule of law for Lebanon's stability in the long term.
The Special Tribunal for Lebanon is a tribunal mandated to hold trials for those accused of being behind the attack in February 2005 that killed 22 people, including Lebanon's former Prime Minister Rafiq Hariri.
The tribunal was inaugurated on March 1, 2009 and is headquartered in the Hague, with an office in Beirut as well.
Voluntary contributions constitute 51 percent of the tribunal's budget, with 49 percent coming from the Lebanese government.