The Liberal Democrats broke the law by making automated phone calls to supporters during their party conference, despite having been warned that their actions would be likely to breach regulations.

The party was served an Enforcement Notice by the Information Commissioner’s Office (ICO), a legal order requiring them to stop the activity within 30 days, after their calls to 250,000 homes on September 17. Recipients were played a recorded message by Nick Clegg, the Liberal Democrat leader.

The ICO launched an investigation after receiving a “number of complaints”. They ruled that the calls breached Privacy and Electronic Communications Regulations, which prohibit unsolicited direct marketing calls to any individual who has not previously consented to them.

Any further phone calls in breach of the Enforcement Notice will constitute a criminal offence and a spokesperson for the ICO said that although the calls were not a criminal offence, they were "illegal" as they breached the regulations.

A Liberal Democrat spokesperson said: “The Liberal Democrats will abide by the ruling from the Information Commissioner's Office. We will not be appealing against the decision.”

The Enforcement Notice reveals that the Liberal Democrats pressed ahead with the phone calls despite a warning from the ICO that the calls would be likely to constitute a breach of the Act.

On the day of the calls Mr Clegg had told BBC Radio 4’s World at One programme that they were for “genuine market research purposes”, which are allowed by the regulations.

However, the ICO subsequently informed a Liberal Democrat official that “on the basis of the verbal summary of the script”, which they had provided to the ICO, “the messages were likely to constitute direct marketing and that, therefore, if sent to those who had not given prior consent, would be likely to involve a breach of the regulations”.

The Liberal Democrats carried out the phone calls that evening despite the warning. A party spokesperson said: “The ICO had not seen the script at that point and we did not have a definitive view from them.”

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