BRITAIN'S alleged violation of the ''three accords'' is to blame for the lack of progress in the Beijing talks on Hongkong political reform, according to Zhang Junsheng, a vice-director of the local branch of the New China News Agency (NCNA). ''If you agree to the three accords verbally, but deviate from them when you are holding the talks, it is impossible for the talks to progress quickly,'' he said. Mr Zhang said early agreement could be reached if both sides upheld the three accords - the Joint Declaration, the Basic Law and other previous agreements and understandings. Another vice-director of the NCNA, Zhu Yucheng, said the political reform proposals put forward by the Governor, Chris Patten, could not be accepted as a basis for the talks. ''We consistently uphold the stance that the Patten package is totally unacceptable in the talks, but there have been some twists in the talks about this question,'' Mr Zhu said. In Beijing yesterday, Chinese Foreign Ministry spokesman Wu Jianmin rejected British assertions that seven diplomatic letters exchanged between China and Britain in 1990 did not constitute agreements. The letters referred to arrangements for the evolution of Hongkong's political system. The lack of progress in the talks will top the agenda of a summit on Hongkong to be held in London on July 1. The Secretary for Constitutional Affairs, Michael Sze Cho-cheung, who is flying to London with Mr Patten for the meeting, said yesterday he would not attend Cabinet discussions on the issue. He denied the British had set a deadline for the negotiations. However, he said an agreement should allow sufficient time to prepare for the 1994/95 elections. Voter registration for the district board elections should begin by next April. ''The voter registration is a process laid down by the electoral law. We can't change that,'' he said.