Voluntary organisations are appealing to the UK Borders Agency to clarify the volunteer category of the new points-based immigration system.
"It is a hideously complicated process that urgently needs reviewing," said Mike Locke, public affairs director of Volunteering England. High levels of bureaucracy could deter people from travelling to the UK to volunteer, he said.
The Tier 5 category of the system requires anyone entering the UK to do voluntary work to obtain a visa. Voluntary organisations receiving them must hold a £400 licence. But there is confusion about what activities are classed as volunteering.
Ignorance about the system has also led to volunteers being turned away. Last week, 14 volunteers from Florida arrived at Gatwick to work at a church in London but were sent home for not having the correct paperwork.
A spokeswoman for volunteering charity CSV, which recruits overseas voluntary workers for some projects, said the rules were delaying service provision while volunteers awaited approval of their visas.
Denise Hayward, assistant director of the Volunteer Development Agency in Northern Ireland, said smaller charities had difficulty paying for the licence, and there was confusion over whether larger groups could act as umbrella bodies to pay the fee for them.
She said her organisation wrote to the UK Borders Agency more than a month ago requesting extra information about the legislation, but had not received a response.