BRITAIN will face a political storm if it fails to secure Chinese backing for the ''single seat, single vote'' election method for the 1995 Legislature Council polls, according to a British source. He said the Hong Kong administration would encounter political difficulty if China agreed to adopt the election method for the district board and municipal council elections - but not for Legco. China has insisted that talks on the electoral methods for the district and municipal governments should be split from the 1995 Legco polls to facilitate an overall deal. The source said the British side had held out great hope for a ''first agreement'' at last week's 16th round of talks so the relevant legislation on the less contentious issues could be put to Legco soon. Should the Government drop the ''single seat, single vote'' election system, it faces the danger of being voted down by Legco, he said. Although the source admitted it was technically feasible to have different election methods for the three sets of elections, it was ''politically difficult''. China is worried that the election method has the danger of domination by large political parties, particularly the United Democrats, and that minor forces such as the Democratic Alliance for the Betterment of Hong Kong will not be able to get a seat. The ''single seat, single vote'' election method was one of the three ''straight-forward'' issues Britain hoped to contain in the ''first agreement'' with China at the just-concluded talks. The other two were voting age and appointed seats to the district boards and the two municipal councils. The British members returned to the territory yesterday and will go back to the Chinese capital for the 17th round on Friday and Saturday. British chief negotiator Christopher Hum said they stuck to the strategy of seeking a ''first-stage agreement'' on the less controversial issues. Asked if he would map out a contingency plan with Governor Chris Patten for a possible breakdown in talks, Mr Hum said they were looking at ''how best we can take the talks forward''. The Secretary for Constitutional Affairs, Michael Sze Cho-cheung, said he would report on the latest talks to Mr Patten today and to the Executive Council tomorrow.