BRITAIN'S chief negotiator for the 1994/95 electoral arrangements, Sir Robin McLaren, will be absent from the most crucial stage of the talks after undergoing an operation in a London hospital for a back problem. Christopher Hum, the assistant under-secretary of state for Asia and the Far East in the Foreign Office, will take his place to head the British negotiating team when the two sides make last-ditch efforts to try to strike a deal. Sir Robin, who headed the British team in the past 15 rounds of talks, is expected to take four to six weeks to fully recovery before returning to work. During Sir Robin's absence, Mr Hum, also an old China hand and intimately involved in Hong Kong issues in his present and previous postings, will take full charge of the talks. Sir Robin left Beijing on Sunday to join Governor Chris Patten in preparation for yesterday's crucial cabinet meeting to map out the British side's negotiating strategy for what is believed to be the final stage of the talks. He was scheduled to return this weekend but will now stay in Britain to recover. Sir Robin had been suffering from back pain for a while and, since the 14th round of talks held in late October, was in particular discomfort during the negotiations. British officials were concerned about the absence of Sir Robin, who has a close rapport with Beijing officials, but Mr Hum has much experience dealing with China. Mr Hum accompanied the British Foreign Secretary, Douglas Hurd, to meet his counterpart, Qian Qichen, to review the progress of the talks in Beijing in July. The first meetings he will attend are the 16th round of negotiations and will be held in Beijing on November 19 and 20. Legislators held different opinions towards Sir Robin's replacement. Meeting Point legislator Fred Li Wah-ming said it would affect the progress of the talks to some extent. ''Compared with Sir Robin, Mr Hum is relatively new to the Chinese. It will take the Chinese side some time to know more about him,'' he said. Noting that it was the chief negotiator who would do all the talking during the negotiations, Mr Li said Mr Hum would need to catch up with developments. Yet, Mr Li believed it would largely be a technical problem. United Democrat Yeung Sum said the replacement would not affect the talks, as Britain had already mapped out its strategy towards the negotiations. He said Mr Hum would not depart from the British stance and he would seek Britain's advice before making any crucial decisions. His view was shared by Liberal Party legislator Henry Tang Ying-yen. Since negotiators had normally followed instructions from their seniors, Mr Tang said the replacement would not affect the talks. Given the fact that Mr Hum had been involved in Hong Kong affairs for quite some time, as well as being a diplomat himself, Mr Tang said Mr Hum was eligible to assume the post of chief negotiator.