The letter comes the day after over 130 captains of industry, including top executives at HSBC bank and mining group BHP Billiton, signed another open letter in The Scotsman newspaper saying "the business case for independence has not been made".
Cameron is on Thursday set to urge Scots not to turn their backs on "one of the oldest and most successful single markets in the world" in a keynote speech to CBI Scotland, a leading business lobby group, in Glasgow.
First Minister Alex Salmond, who is leading the "Yes" to independence campaign, argues that the British government's economic policies are unfairly skewed in favour of London and southeast England.
He said business leaders were "waking up to the opportunities of independence". "With full control over economic powers, we have the opportunity to tailor economic policy to our needs," Salmond added.
Read more here: Business Spectator