Pages on charity websites that include a request for donations will have to comply with the Advertising Standards Authority's codes from next week.
The codes, which will also apply to microsites and pages controlled by charities on social networking sites such as Facebook, include the rule that advertising must be responsible and must not mislead or offend.
The extension of the codes to the parts of all websites that involve sales and donations has been prompted by an increase in complaints about such sites.
The extended remit will not cover pages that promote causes or campaigns but do not include a request for a donation.
Charities that do not adhere to the code after being asked to do so by the ASA could be added to a blacklist on its website. They could also have their paid-for search advertising removed and be included in ASA online advertisements highlighting their continued non-compliance.
"We would encourage anyone who is unsure to get in touch with us," an ASA spokesman said. "We're not trying to catch people out."
He said the ASA was offering training for a fee and a free advice service for charities.
Organisations can send materials to check whether they could be caught under the code to firstname.lastname@example.org, he said.