The Charity Commission has said it will need extra resources to deal with the estimated 30,000 excepted charities that are due to join the register of charities by 2021.
Churches and other charities that have incomes of £100,000 or less, are "wholly or mainly for public religious worship" and linked to certain Christian denominations do not have to register with the commission. But after changes were introduced in the Charities Act 2006, that exception will end in 2021.
A spokeswoman for the commission said it estimated that there were 30,000 excepted church charities, five times the 5,949 charities it registered in 2012/13.
"It is unrealistic for every single member of every single currently excepted denomination to be registered immediately in 2021," she said.
She said the commission had begun work on a "proposal to make sure that we are properly equipped to manage demand", but could not start this yet because it did not have the resources required. She did not specify how much money would be needed.
"We are aware that some charities have concerns about this," she said. "But we are also aware that there is impatience from others who recognise the benefits of registration and would like to join the register sooner, on a voluntary basis."
The commission said it would be likely to provide model constitutions for such charities.
In a debate on the regulator’s effectiveness in the House of Lords last month, the Lord Bishop of St Albans said that "unless some queuing system is agreed in advance, at the end of the seven-year extension there is a real possibility of a logjam".
He also said the number of excepted organisations might drop by 2021 because inflation meant more charities reached the £100,000 threshold.