Cameron pledged to do all he could to keep the United Kingdom together and said he would head north to Scotland on Wednesday to join the fray.
"In the end, it is for the Scottish people to decide, but I want them to know that the rest of the United Kingdom - and I speak as prime minister - want them to stay."
Cameron's move made clear that the break–up of the United Kingdom - previously thought to be a pipedream - was now a distinct possibility. His spokesman said Scotland's blue and white flag would be flown over Cameron's London residence in Downing Street until the vote next week.
Britain's main political parties meanwhile scrambled to shore up the 307-year union by pledging more autonomy to Scotland.