The ICO investigated emails sent from the Conservatives in the name of Boris Johnson during the eight days after he became prime minister in July 2019.

The ICO concluded that 51 emails had been sent to people who did not want to receive them. The emails were addressed by name and promoted the party’s political objectives.

The investigation found the Conservatives failed to retain clear records of the basis upon which people had consented to receive marketing emails, as required by law.

The Conservatives sent out almost 1.2 million emails between 24 July and 31 July 2019, and the ICO was unable to ascertain what proportion were validly sent.

The ICO concluded the party did not have the necessary valid consent for the 51 marketing emails received by the complainants. The party also failed to ensure records of those who had unsubscribed from its marketing emails were properly transferred when it changed email provider.

Almost 23 million emails were also sent by the Conservatives in the 2019 general election, which generated a further 95 complaints to the ICO. The ICO said the complaints were likely to have resulted from the party’s failure to address the original compliance issues identified in July 2019.

Stephen Eckersley, director of investigations, said: “Getting messages to potential voters is important in a healthy democracy but political parties must follow the law when doing so. The Conservative party ought to have known this, but failed to comply with the law.

“All organisations – be they political parties, businesses or others – should give people clear information and choices about what is being done with their personal data. Direct marketing laws are clear and it is the responsibility of all organisations to ensure they comply.”

A Conservative party spokesperson said: “We have accepted this fine from the ICO. We have since reviewed and improved our processes and are fully compliant with all prevailing data protection and electronic marketing legislation.”