The website of the National Childbirth Trust has been hacked with the loss of more than 15,000 names and passwords.
A spokesman for the NCT, which provides support and advice for parents and prospective parents, said the hackers, who breached the site on Wednesday, had not stolen or compromised any financial information.
He said the NCT had reported the attack to the police and the Information Commissioner’s Office.
"NCT has suffered a data breach that, regrettably, has caused some users of our website to have their website registration details compromised," he said.
"These details are limited to their email address, username and an encrypted version of the password that they created to register on the site. We stress that no financial or personal details are held as part of this data so no financial or personal details have been compromised.
"The breach occurred and was discovered on Wednesday, upon which we contacted everyone affected, telling them about the breach and advising that they change their username and passwords."