By Guy Faulconbridge and Angus MacSwan LONDON/EDINBURGH (Reuters) - British financial markets tumbled on Monday after an opinion poll showed for the first time this year that Scots may vote for independence in a referendum next week, breaking up the United Kingdom. The survey prompted concern bordering on panic among Britain's ruling elite, with Prime Minister David Cameron's Conservative-led government promising proposals this week to grant Scotland greater autonomy if it stays. Cameron's job would be on the line if Scots vote on Sept. 18 to secede, less than eight months before a national election planned for May. His spokesman said on Monday the government was not making contingency plans for the possibility of Scottish independence. Sterling fell more than 1 percent - its biggest one-day drop in 13 months - to $1.6141, long-dated government bonds tumbled and 3.5 billion pounds ($5.7 billion) was wiped off the market value of six London-listed companies with large exposure to Scotland. Click here to read full news..