On Monday, the regulator published guidelines for companies, charities and campaigners ahead of the formal referendum period, which begins on Friday and lasts until the vote on 23 June.
The rules state that up to £10,000 can be spent on campaigning activities intended to promote a particular outcome in the referendum.
Companies or individuals must ask the commission to become a registered campaigner if they wish to employ a larger budget than £10,000 and must then follow its rules on donations and loans as well as reporting details of all spending.
The regulator set out guidelines on what would be considered referendum spending and said that many, but not all, "business as usual activities" would not meet the test for referendum spending.
In order not to count, business activities must not be intended to bring about a particular outcome in the referendum. For example, a company’s annual report may mention the potential risks of a particular outcome but if the commentary in the report was intended to influence the choice voters make on 23 June, then it would be classed as referendum spending.
If a business offered advice to clients in the run up to the referendum, that would be considered professional advice, but only if it was within the remit of the service that business provides, rather than to directly influence the outcome.
The regulator also gave guidance on producing research or risk analysis.
It said reports would be considered referendum spending if they compared one outcome with the other or made a value judgement about the preferred outcome, rather than striking a neutral tone. Surveys and polls carried out by companies or organisations that "ask leading questions with the aim of obtaining a particular result" would also fall foul of the guidelines on referendum spending, the commission said, but if they were for internal purposes only, they would not.
Similarly, events held by companies to promote a specific result would also be considered referendum spending, unless speakers on both sides of the debate were given equal opportunity to participate.