Malaysia has long outlawed books deemed "deviant" from mainstream Sunni Islam, but a ban on a book of essays by public intellectuals on Islam's role in its constitutional democracy has drawn wide criticism for pandering to Islamists.

The book, Breaking The Silence: Voices Of Moderation - Islam In A Constitutional Democracy, was produced by a group of prominent Muslims who in 2014 dubbed themselves the Group of 25, or G25.

The ban comes at a time when questions are raised over the reach of Malaysia's Islamic authorities and its impact on government bureaucracy, and whether the voices of moderate Muslims are being cast aside and replaced by those with deeply conservative views.

The book was launched in December 2015 by Umno's longest-serving MP Tengku Razaleigh Hamzah. In the book's foreword, former prime minister Abdullah Badawi wrote that he hoped it would "encourage an informed and rational dialogue on the ways Islam is used as a source of public law and policy in multiracial and multi-religious Malaysia yet within the letter and spirit of the federal Constitution".

The essays include those by constitutional lawyer Shad Saleem Faruqi, director of pro-government think-tank Just World Chandra Muzaffar, Universiti Malaya law lecturer Azmi Sharom, and former executive director of Sisters in Islam Ratna Osman.

G25 is seeking an explanation from the Home Ministry for the ban, calling it "an obvious action meant to suppress free speech".

"It is very disheartening if the government is giving in to Islamists and conservatives. They see us as secular and unfortunately, to them, secular is a bad word meaning anti-Islam," G25 spokesman and former judge Noor Farida Ariffin told The Sunday Times.

The ban, made under the Printing Presses and Publications Act on June 14, was gazetted on Thursday.

According to the gazette signed by Deputy Prime Minister Ahmad Zahid Hamidi - who is also Home Minister - the book "is likely to be prejudicial to public order, likely to alarm public opinion, and likely to be prejudicial to public interest is absolutely prohibited throughout Malaysia".

Centre for Public Policy Studies chairman Ramon Navaratnam has challenged the ministry to "state clearly the evidence of any resulting public disorder and openly explain whose public opinion and public interest has been prejudiced or badly hurt by this learned book".

Critics of the ban said it was contrary to Prime Minister Najib Razak's international push for moderation to hold sway in the face of religious extremists.

The government's own Human Rights Commission said it was "aghast" at the ban as most of the book's chapters focused on moderate Islam as advocated by PM Najib.

And opposition stalwart Lim Kit Siang said: "What is the use of the Prime Minister launching the international initiative of a Global Movement of Moderates... if the Cabinet allows the forces of extremism and intolerance to rear their ugly heads by jettisoning the constitutional principles of moderation, such as arbitrarily banning the G25 book."

At a conference of Islamic scholars last Friday, Datuk Seri Zahid said: "I am loyal to Najib. I believe in his idea of moderation and I will assist him to translate his idea of moderation into the system."

The G25 says "overreach" by the religious authorities undermines Malaysia's secular Constitution.