Donald Trump has warned Alex Salmond that going ahead with an offshore windfarm would do more damage to the region "than any event in Scotland's history".
The US business tycoon has previously spoken out against plans for an offshore wind farm in Aberdeen Bay, near his golf resort in Aberdeenshire, Scotland.
In his letter to the SNP leader, Trump said the project amounted to "insanity" and would mount an international campaign against the proposed turbines.
"As a matter of fact, I have just authorised my staff to allocate a substantial amount of money to launch an international campaign to fight your plan to surround Scotland's coast with many thousands of wind turbines."
He wrote: "Please understand that I am doing this to save Scotland."
Trump warned Salmond in the letter: "With the reckless installation of these monsters, you will single-handedly have done more damage to Scotland than virtually any event in Scottish history."
A Scottish Government spokesperson said: “An application for consent for the European Offshore Wind Deployment Centre has been submitted to Marine Scotland, and we are currently considering the views of consultees, interested parties, and the public.
"Scottish waters are estimated to have as much as a quarter of Europe's potential offshore wind energy. A recent study suggests that harnessing just a third of the practical resource off our coast by 2050 would enable us to generate enough electricity to power Scotland seven times over. An independent Scotland will be able to take full responsibility for this renewables revolution, along with the investment and thousands of jobs it brings.”