Hugo Swire, a minister of state with the Foreign Office, summoned Nadhavathna Krishnamra "to raise concerns about the investigation," according to a government statement Monday.
Two arrested migrant workers from Myanmar have said they were forced to confess to the September 15 murder of backpackers Hannah Witheridge, 23, and David Miller, 24, on the island of Koh Tao.
Police say the confessions are valid, and have denied reports of a retraction.
Authorities have also been criticised for failing to cordon off the crime scene or restrict traffic to and from the island in a timely manner.
Swire said there was "a real concern in the UK about how the investigation has been handled by the Thai authorities," according to the statement.