Britain's Moslem and Jewish communities have joined in an uneasy and informal alliance to protest what both see as a government threat to their religious freedoms.

While wary of any official ties, leaders of the country's 380,000 Jews and 2 million Moslems have held informal consultations and appeared side by side on public platforms to speak out against proposals that would make illegal certain religious practices common to both.

In a recent report to the Agriculture Ministry, a government-appointed committee has recommended that animal slaughter procedures, known as shechitah to Jews and halal to Moslems, be outlawed as causing undue stress and pain to livestock.

The recommendations are backed by this country's powerful animal rights lobby and the national veterinary association. According to John Douglass, head of the farm animal department of the Royal Society for the Prevention of Cruelty to Animals, the religious slaughtering methods are "tantamount to cruelty."

In most respects, shechitah and halal procedures are identical to slaughter in secular abattoirs, where livestock are killed by severing major blood vessels. But modern practice calls for the animal to be stunned, or rendered insensible by a massive blow to the head with a captive-bolt pistol or electric shock, before the cut is made.

Stunning before killing makes meat unacceptable to both Islam and Judaism. Most interpretations of their separate scriptural laws hold that meat is not fit for consumption unless it is "undamaged" at the time of slaughter.

Many developed countries require stunning, in the belief that the blow is less painful to the animal than the cut. But it is a question over which scientists have differed, with some researchers concluding that stunning causes more pain. The United States, Canada and all 10 members of the European Community specifically exempt religious slaughter methods from stunning regulations.

But the committee, many of whose previous recommendations on farm animal welfare have become law, has asked that Britain's exemption be withdrawn.

"The up-to-date scientific evidence available and our own observations," its report said, "leave no doubt in our minds that religious methods of slaughter, even when carried out under ideal conditions, must result in a degree of pain, suffering and distress which does not occur in the properly stunned animal."

It has recommended that stunning be required by 1988 for all of the estimated 91,000 cattle and 1.5 million sheep and goats slaughtered each year in halal and shechitah abattoirs here.

Without the process that produces kosher meat, Rabbi Bernard Berkovits argued during a symposium held early this month under the auspices of London's famed Smithfield meat market, observant Jews here would be forced to become vegetarians. The banning of shechitah would amount to religious discrimination, said Berkovits, registrar at the court of the chief rabbi to Britain's mainly orthodox Jewish community.

Sharing the Smithfield platform with Berkovits was Imam Mohammed Oveisi, of London's Moslem Institute. Halal meat, sold in special stores throughout the country, is not simply a Moslem preference, he said. It is Moslem law.

Both communities, Moslems particularly, have had to fight occasional battles here against persecution and discrimination. But although there is little history here of public enmity between the two minorities, neither has there been much for them to agree on.

Both now express some discomfort at being on the same side of a religious issue. Leaders hasten to explain that while shechitah and halal methods agree on the key requirements for undamaged meat and procedures ensuring a basic kindness to animals, separate rituals are involved.

"We have had some informal contact," Berkovits said in an interview. Both communities, he said, believe their procedures "are binding under religious law."

Moslems view the support of the smaller but better organized Jewish community as a definite asset.

"There are members of Parliament who are Jews -- at least 70 or 80 of them," said Syed Pasha, general secretary of the umbrella Union of Moslem Organizations of the U.K. and Eire. "There is not a single Moslem member."

According to Berkovits, however, only 26 of Parliament's 650 members are Jews, and they have no power as an effective lobby.

Although they limit their public arguments against the ban largely to conflicting scientific evidence over animal pain, leaders of both communities believe more subjective elements are involved. Both are aware of the devoted attachment to animals in Britain, where many believe that a house without a dog or cat is not a home.

"I recall a couple of years ago a big headline in one of the national newspapers about a woman who had thrown her dog out of the window," Berkovits said. "On the bottom of the same page was a small story about a woman who had killed her baby."

Douglass, of the RSPCA, described such arguments as nonsense. The number of court cases brought by his organization, he said, indicated that Britons can be just as cruel to animals as to each other.

More seriously, Moslems and Jews say they detect overtones of racism, influenced by the National Front, a neofascist organization that has promoted the religious slaughter ban in its literature.

"If it were purely the animal groups, it wouldn't have come to this," Pasha said.

Religious slaughter has been a recurrent theme in Britain ever since waves of South Asian immigrants began arriving here in the 1950s, and the front's efforts are fueled by regular reports in the tabloid press of "barbaric" Moslem practices.

Last year, for example, employes at the Iranian Embassy here caused major headlines and a minor diplomatic incident when they slaughtered a sheep on a London curbside in full view of the neighbors. In September, Anglo-Saxon sensibilities were further offended in the South Yorkshire city of Rotherham by an Asian grocer who celebrated an Islamic holiday by slaughtering 29 sheep in his back yard.

But Moslem spokesmen say that these incidents are easily avoided exceptions that have nothing to do with what goes on in the abattoirs. The real issue, they and Jewish leaders agree, is one of religious freedom.