The email was received by Remain supporters who signed up to receive information from the ‘Stronger In’ campaign during the referendum last year. The organisation, which lobbies for the UK to stay in the EU, is now known as ‘Open Britain’.
The email offered supporters the chance to buy ‘How to Stop Brexit’ at a discounted price of £4.49.
There are strict rule about how information can be used by third parties under the 1998 Data Protection Act.
The Information Commissioner’s Office is responsible for policing data laws and has the power to fine any individual or organisation that breaches them.
One of the underlying principles of the legislation covering data use is that email addresses cannot be used for any purpose other than the one they were originally intended for without the consent of the individuals who provided them.
The Information Commissioner has received at least one complaint about Clegg’s use of Open Britain’s database to promote his new book, which is believed to have sold poorly.
In the email Clegg wrote: “Brexit is far more complicated and will be far more painful than Leave campaigners and Ministers have ever admitted” before going on to plug his book.
‘Stronger In’ rebranded itself as Open Britain after the vote last year.
It is run by James McGrory, who worked as special advisor to the former Liberal Democrat leader when he served as Deputy Prime Minster in the Cameron coalition government.
One political observer said: “The fact that Nick Clegg is using Open Britain to market his book looks slightly desperate. It is also completely inappropriate. The group’s supporters signed up to back a political campaign – not to receive marketing material”.