Charities have been told that the possible loss of European Union funding does not justify an organisation campaigning for a Remain vote in the referendum.
Guidance issued to trustees makes clear that political activity such as campaigning for either side in the June 23 vote "can only ever be undertaken in support of your charitable purposes".
The Charity Commission guidance warns there could be "reputational risks" for charities which decide to get involved in the political battle.
Warning that the potential loss of Brussels funds is not on its own a reason for campaigning, the guidance said: "Many charities are funded by the EU or its institutions, and conditions are usually attached to the funding.
"For charities that are in direct receipt of such funding, the possibility of a loss of funding will clearly be an issue. However, knowing that the outcome of the referendum could result in a loss of funding would not in itself justify political activity directed at the UK remaining in the EU.
"The key issue is how remaining in or leaving the EU would affect your charitable purposes and the ability of your charity to continue its work.
"The extent of the charity's engagement must be proportionate to the issue involved. Trustees must consider issues such as the risk posed to the charity by the loss of funding and the ability to replace the funding that is at risk of being lost.
"If your charity does get involved in any political activity connected with the referendum, you should ensure that, during such involvement, you publicly acknowledge the source of your funding so that the reasons for your involvement can be fully assessed.
"If you do not do so, this could seriously undermine and detract from the quality of your contribution to these very important issues and may attract regulatory scrutiny by the commission.
"In those exceptional circumstances where a charity considers the outcome of the referendum itself is likely to affect directly, positively or negatively, the delivery of their charitable purposes, full transparency about funding is especially important."
The commission warned charities it will be closely monitoring the situation and will take action where the guidance is breached.
Kenneth Dibble, the commission's director of legal services, said: 'Political activity by a charity can only ever be undertaken in support of its charitable purposes.
"This guidance sets out the rules charities must follow should they be considering engaging in the EU referendum.
"The guidance clearly says that there are reputational risks for charities which engage in political activity on the EU referendum. Trustees must therefore consider the guidance carefully before involving their charities."