Via the Guardian:
The foreign secretary applied for a PII certificate on 7 February. He argued that if secret evidence were revealed it might damage "national security and/or international relations". He gave no further details.
Ben Emmerson QC, acting for Marina Litvinenko, vehemently opposed Hague's request and accused the government of a "cover-up". He said Hague and David Cameron were seeking to suppress material not for reasons of intelligence but so as not to damage Britain's trade interests with Moscow. The government, he told the hearing, was in effect "dancing to the Russian tarantella".