POLITICAL relations between Beijing and London have sunk to a new low over Hong Kong, with an angry exchange of letters between senior figures in the two capitals, a British source has revealed. Beijing has launched a ''coded'' attack on the House of Commons Foreign Affairs Committee (FAC) investigation into Sino-British relations and their effect on Hong Kong. The committee is preparing a report expected to firmly back Governor Chris Patten's reforms. The influential British parliamentary committee has responded by rejecting the Chinese attack. An angry letter from the Foreign Affairs Committee of China's National People's Congress - a copy of which has been obtained by the Sunday Morning Post - claims its British counterparts have ''grossly interfered in the internal affairs of China and severely trampled upon the norms governing international relations'' by investigating Beijing's human rights record in Tibet. It warns of the consequences for ''bilateral relations'' of the British parliamentarians' investigation, and threatens that it will ''undermine the normal exchanges between the two parliaments''. ''The Chinese side has done nothing to harm Britain and its Parliament. However, in disregard of the sound relations between the two parliaments, a small number of British MPs have grossly interfered in China's internal affairs. This has not only seriously hurt the national feelings of the Chinese people but also contravened the position of the British Government,'' the letter alleged. Although ostensibly concerning the committee's investigation of human rights in Tibet, one senior member said they were taking the letter as a coded attack on their inquiry into Hong Kong, which recently heard evidence from Mr Patten and will this week question Foreign Secretary Douglas Hurd on the issue. ''Make no mistake about it. This letter is not just about Tibet. It's a message of how bitter the Chinese side feels about Hong Kong,'' the committee member said. ''They know we're going to come out in support of Chris Patten and insist in our report that there must be human rights safeguards in Hong Kong . . . this is a coded message to us about how they feel over our stand on Hong Kong. ''Our view is that the Chinese record on human rights in Tibet is one way of judging what they might do in Hong Kong,'' he added. A source said the British parliamentary committee had drafted a response rejecting the accusations and stressing its right to investigate China's human rights record. Beijing's letter was provoked by a December 8 hearing which called as a witness Tashi Wangdi, foreign minister for the Tibetan government in exile, headed by the Dalai Lama. China does not recognise the Dalai Lama's government, and frequently protests against the activities of its members in foreign countries. ''The Foreign Affairs Committee allowed its hearing to be used by the so-called 'witness' to shamelessly deny China's sovereignty over Tibet and flagrantly advocate the independence of Tibet. The political motive of the committee in doing so was all too clear,'' the letter said. ''Such an act of the committee constituted open support of and connivance with the political activities of the Dalai clique aimed at opposing the Chinese Government and splitting the motherland.''