The British government sought on Tuesday to limit the information it would disclose at a planned inquest into the death of Alexander V. Litvinenko, a former officer in the K.G.B. who succumbed to radiation poisoning in London more than six years ago. The coroner hearing the case said that it may now be postponed. “Due to the complexity of the investigation which necessarily precedes the hearings,” the coroner, Sir Robert Owen, said, “it may not be possible to adhere” to the planned May 1 start date for the hearings. The inquest would be the first — and probably the only...