The Special Tribunal for Lebanon could release its findings on the 2005 assassination of former Lebanese Prime Minister Rafik Hariri as early as the end of this year.
Employees with the STL were attacked last week after they tried to enter a women's clinic in Beirut. Hezbollah, which is believed to have played a role in the Hariri assassination, said the investigation at the clinic went beyond the scope of the STL's authority.
British Foreign Office Minister Alistair Burt said London was "extremely concerned" about the incident, which he said amounted to meddling in the STL's investigation.
"The U.K. is a firm believer in international justice," he said in a statement. "The Special Tribunal must be able to conclude its work freely and independently."
The STL in response to Hezbollah's allegations said claims that it was overstepping its "ethical, religious and humanitarian norms are false."
The tribunal said no women were present during the visit last week in order to avoid any inconvenience.
"Any call to boycott the tribunal to prevent it from fulfilling its mandate is a deliberate attempt to obstruct justice," a statement read. "The office of the prosecutor will not be deterred by violence and expects that the cooperation will continue with the tribunal in accordance with the STL statute and the Lebanese laws."