Mr. Litvinenko died in November 2006, three weeks after drinking tea at a meeting with Mr. Kovtun and Mr. Lugovoi at an upscale hotel in central London. Mr. Kovtun and Mr. Lugovoi, who both deny the allegations, would be arrested if they traveled to England, but, in March, Mr. Kovtun signaled his readiness to testify by video link.

Image Dmitri V. Kovtun refused to testify in the inquiry. Credit... Pavel Golovkin/Associated Press

Late last week, however, Mr. Kovtun began to express reluctance to testify unless Russian investigators released him from a confidentiality agreement in connection with a separate inquiry in Moscow.

Shortly before the 9 a.m. deadline for him to testify on Tuesday, Mr. Kovtun informed Judge Owen that he would not appear in the video link. British officials have said that they fear Mr. Kovtun’s actions are part of a maneuver to discredit the inquiry. His testimony would have been the first and only oral evidence from Moscow, either by suspects or by Russian officials.

“This unhappy sequence of events drives me to the conclusion either that Mr. Kovtun never in truth intended to give evidence and that this has been a charade,” Judge Owen said on Tuesday, or “alternatively, if he has at some stage been genuine in his expressed intention to give evidence, obstacles have been put in the way of his doing so. In either case I will not have the progress of the inquiry further disrupted.”

Mr. Kovtun’s failure to appear cleared the way for lawyers to offer their final statements before Judge Owen begins preparing a report expected by the end of the year.