In the final minutes of his time in the Oval Office, just before he left for his successor’s inauguration, Barack Obama slipped a letter inside the top drawer of the Resolute Desk.

In it he offered Donald Trump, the incoming President, his congratulations, a rather pointed guide to the American constitution and some words of advice.

“Congratulations on a remarkable run,” he wrote, according to a copy of the letter obtained by CNN. “Millions have placed their hopes in you, and all of us, regardless of party, should hope for expanded prosperity and security during your tenure.

“This is a unique office, without a clear blueprint for success, so I don't know that any advice from me will be particularly helpful. Still, let me offer a few reflections from the past 8 years.”