During a speech in London's Olympic Park, Cameron said the entire United Kingdom would be "deeply diminished" if Scotland chose to leave, the BBC reported Friday.
"This is a decision that is squarely and solely for those in Scotland to make. I passionately believe it is in their interests to stay in the U.K.," Cameron said. "That way, Scotland has the space to take decisions while still having the security that comes with being part of something bigger."
The prime minister said he had been advised to not take part in the debate "but frankly, I care far too much to stay out of it," Cameron said.
"This is personal," he said.
The Scottish National Party said Cameron's decision to make a speech on the matter in London and not in Scotland was "cowardly."