CHINA yesterday launched a belated attack on the relocation of the naval base from HMS Tamar to Stonecutters Island, while Britain claimed the Chinese side had previously agreed to it. Although Britain said on March 26 the navy was starting the two month process of moving the base, China's accusation, that London was taking a unilateral decision for which it must bear all consequences, came only yesterday morning during a regular meeting of representatives of the Joint Liaison Group (JLG). It was made public in a news despatch from the local branch of the New China News Agency in the afternoon. The British Foreign Office spokesman in Hongkong, Mr Chris Osborne, said it was the first objection to the shift that Britain had received. It is understood that China agreed at a JLG meeting in September 1991, that Tamar naval basin could be filled in and the naval facilities moved to Stonecutters Island. The $350 million relocation would leave 4.4 hectares of land to the east of the Prince of Wales Building available for redevelopment as part of the Central and Western Reclamation Project. It was believed the only matter unresolved in JLG negotiations was the future of the Prince of Wales Building, the headquarters of the British garrison. The British side decided the building would be left intact pending further discussion with China. China has repeatedly voiced its objection in private to the British proposal to turn over several military sites, including the Tamar site, to residential or commercial use. The NCNA despatch quoted a spokesman from the Chinese side as saying China resolutely opposed any unilateral disposal of military lands. The spokesman said it was ''absolutely unacceptable'' to China that Britain made such a move before reaching an agreement with the mainland on the change of land use. He said: ''If Britain is bent on its own way, it has to be responsible for all consequences which arise.'' Mr Osborne said he had no idea why China had opposed the move or why the statement was delivered at such a time. Stressing that China had been consulted throughout regarding the relocation exercise, Mr Osborne said: ''It [the relocation] is not a move that can be delayed any longer.'' According to the British garrison's plan, the relocation exercise would be completed by May 18. A Joint Services public relations officer refused to comment. The military base will be known as the Prince of Wales Barracks after the navy's switch and the headquarters of the British Forces in Hongkong will remain on the site until 1997. But the Chinese JLG statement said the Joint Declaration and the Basic Law stipulated that Hongkong's defence matters should be the responsibility of the Chinese central government. ''To ensure a smooth transition, the military sites being used by the British Forces have to be handed over to the Chinese garrison for the defence of Hongkong after 1997,'' it said. It also stressed that China recognised that social and economic development would create a need for land and that it was willing, through consultation, to turn several military sites over for other uses. For example, the Burma Lines garrision in Fanling was handed over to the police tactical unit last year.