The Fourth Protocol by Frederick Forsyth

Professional thief Jim Rawlings breaks into the apartment of a senior civil servant, and unintentionally discovers stolen top secret documents. Although one of the most notorious thieves in London, he is enough of a patriot to anonymously send the documents to MI5 so that they might locate the traitor. In Moscow, British defector Kim Philby drafts a memorandum for the Soviet General Secretary stating that, if the Labour Party wins the next general election in the United Kingdom (scheduled for sometime in the subsequent eighteen months), the "hard left" of the party will oust the moderate populist Neil Kinnock in favour of a radical new leader who will adopt a true Marxist-Leninist manifesto, including the expulsion of all American forces from the United Kingdom and the country's withdrawal from and repudiation of NATO. In conjunction with a GRU general, an academic named Krilov, and a master strategist, Philby devises "Plan Aurora" to ensure a Labour victory by exploiting the party's support for unilateral disarmament - although it is noted that the strategist, a nuclear physicist and chess Grand Master, has come up with most of the plan's strategy.