Standard & Poor's downgraded Greece's credit rating deeper into junk territory on Monday, saying the government's decision to hold a referendum brought it closer to an exit of the eurozone.

"We interpret Greece's decision to hold a referendum on official creditors' loan proposals as a further indication that the Tsipras government will prioritize domestic politics over financial and economic stability, commercial debt payments, and eurozone membership," said S&P, cutting the rating to 'CCC-' from 'CCC'.