By Josh Beckerman 

Standard & Poor's Ratings Services has downgraded Puerto Rico to CCC+ from B and placed the rating on CreditWatch with negative implications.

The ratings firm attributed the downgrade to "our view that the commonwealth's market access prospects have further weakened and Puerto Rico's ability to meet its financial commitments is increasingly tied to the business, financial, and economic conditions on the island."

S&P believes "budget pressures are exacerbated by current weak economic trends and high debt levels."

Puerto Rico's government may run out of money and have to shut down in the next three months, with a "devastating impact" for the U.S. commonwealth's economy, officials at the island's Government Development Bank warned in an April 21 letter.

Puerto Rico has more than $70 billion in debt and is struggling with a weak economy and high unemployment.

Write to Josh Beckerman at josh.beckerman@wsj.com