The Duke of Cambridge occasionally receives copies of confidential cabinet documents, the BBC has learned.
A Cabinet Office spokesman said that, as a future heir to the throne, it was “appropriate that he is regularly briefed on government business”.
It was revealed on Tuesday that Prince Charles receives such material routinely – as does the Queen.
Campaign group Republic said there was “no good reason” why Prince William also received the information.
Earlier this week, Republic received four chapters of the Cabinet Office’s “precedent book” – released after a three-year freedom of information battle.
The book shows Prince Charles, the Queen, ministers and a handful of others get papers from cabinet and ministerial committees.
Junior government ministers do not receive such access to the documents.
Of Prince William’s access, the group’s chief executive Graham Smith said: “There is no mention of this access in the documents released this week. It appears to be a free for all.
“We support Labour’s call for an inquiry, so we can know how much information is being handed to which royals.”
Following the initial discovery about Prince Charles’ access to the documents, Republic wrote to Prime Minister David Cameron calling on him to end the practice of sharing the documents with the Prince of Wales.
Labour MP Paul Flynn called for a parliamentary inquiry and said the access made the prince the country’s “best-informed lobbyist”.
Shadow energy and climate change minister Clive Lewis has also called for a parliamentary review.