Pakistan's pro-military government, a key ally in what Washington calls its "war on terror", has been asked by the United States to send around 10,000 soldiers to Iraq to help secure the post-war peace.
Fazal-ur-Rehman, a central leader of the main Islamic alliance, said a council of top religious scholars had been set up to issue the decree, or fatwa, next week.
"A council has been formed to issue a fatwa that serving of Muslim troops under the command of the United States is un-Islamic," he told a news conference.
Musharraf under fire
Rehman said military ruler President General Pervez Musharraf had "no constitutional, religious and legal right to send troops to Iraq."
"We condemn the government's decision to send troops to Iraq."
Muslim cleric Fazal-ur-Rehman
Islamabad, which backed the United States in its war against the Taliban in neighbouring Afghanistan, says it has agreed in principle to send troops to Iraq.
But Musharraf said last month he would prefer his troops to serve under the auspices of the United Nations, the Organisation of Islamic Conference or another legitimate international body.
"We condemn the government's decision to send troops to Iraq," Rehman said.
Occupiers try to share burden
The condemnation by the Islamic coalition came as a senior British government minister was quoted on Saturday as saying that Britain wanted to make it easier for countries including India, Pakistan and Turkey to join a UN-backed multilateral peacekeeping force.
International Development Secretary Baroness Valerie Amos told the Daily Telegraph that the United States and Britain were ready to support a new UN resolution that would give these countries the domestic cover they needed to contribute troops.
Muslim groups made stunning gains in last October elections by tapping anti-American sentiments over the US-led war in neighbouring Afghanistan. They strongly oppose Musharraf's close ties with the United States.
Anti-US feelings run deep in Pakistan
The Islamists are also locked in a long-running standoff with Musharraf over the military's dominant role in politics.
They want him to step down as chief of army staff or as president and withdraw controversial constitutional amendments he made before returning civilian rule in the country.
The constitutional changes, or Legal Framework Order (LFO), empower Musharraf to dismiss an elected parliament and appoint a military-civilian National Security Council to oversee affairs of the government.
"There is no change in our principled stand that LFO is not part of the constitution," Rehman said. "It cannot be incorporated into the constitution through force."