A MAJOR government policy allowing expatriate officers to switch to local terms was blocked by legislators yesterday in an unprecedented move. The approval of a private member's bill which upsets the Government's wishes and blocks all applications up to next April came from the combined weight of the United Democrats and the Liberal Party. At the end of the two-hour debate, the acting Secretary for Civil Service, Stuart Harbinson, claimed legislators had brought politics into the issue. He said it was a prime example of ''back-seat driving'' by legislators to meddle in the Government's appointment of its own officers. ''I appeal to members not to start on this slippery slope,'' he said. He said the bill, which sought to block all applications for switching terms until April 20, was tantamount to further prolonging the out-of-date ''differential treatment'' of locals and expatriates. He said the Government would have no choice but to extend officers' contracts on overseas terms, instead of local terms, as the policy intended. Mr Harbinson denied an earlier rumour that Governor Chris Patten would refuse to sign the bill in an attempt to veto it. He said the legislators' decision would be respected. Most legislators charged that it was the Government which stirred up the controversy by suddenly allowing overseas officers to switch to local terms. Pro-China legislator Tam Yiu-chung, who moved the bill in his name, said his intention was to urge the Government to map out a long-term solution for expat and local civil servants. ''We have no intention of creating any chaos or of challenging the authority of the Government.'' United Democrat legislator Cheung Man-kwong said the bill was neutral as it sought only to freeze the policy until April. Until yesterday, the Government had approved two and rejected 12 applications to switch to local terms, with 91 pending.