New security guidance is being sent to every church in the UK following the murder of French priest Father Jacques Hamel by suspected Islamist militants.
The organisation National Churchwatch is advising that all church buildings should have CCTV.
A single public entrance that can be slammed shut against attackers and personal attack alarms for key personnel are also advised.
Churches in smaller villages are at greater risk of attack, it warned.
The advice states: "If someone produces a weapon during a service (and this will usually be a knife or blunt instrument) then you will need to get people out of the church. Do not let them just sit there and watch what is going on.
"They are at risk. It only takes seconds for someone to stab a large number of people as we have seen in various attacks around the country.
"Think about how you would get the congregation out. This is especially important in larger churches where the children may be separated from their parents in a Sunday School."
National Churchwatch's director, Nick Tolson, said eight clergy had been murdered - all in knife attacks - since 1996.
He said the risk of an attack had risen since the murder in July of Father Hamel, who had his throat cut when two men stormed his church in a suburb of Rouen, northern France, during Mass.
"My experience tells me it won't be a large church or cathedral," he said.
"It'll be a small church where there's not much around and the police are a long way away."
The Church of England said it had not heard of any reported instructions to clergy to wear regular clothes.
"We do not know of any instruction or advice for clergy not to wear dog collars," said a spokesman.
Christian leaders have said clergy should never resort to disguise.
The leader of Coptic Orthodox Christians in the UK, Bishop Anba Angaelos, said: "While potentially making us targets our visible presence is needed as reconciling, peaceful resilience for the world."
The Anglican archdeacon of London, the Venerable Luke Miller, said: "Clerical dress in public is a sign of our mission of the Prince of Peace. We must not be terrorised out of it."