A widely-used political campaigning tool employed by Boris Johnson, Donald Trump, and the SNP has been buying data on British voters from a company accused by Facebook of violating its users' privacy.

NationBuilder, a voter management system which was used by up to 200 groups in the 2017 general election, paid an American company called FullContact to scour the internet for social media accounts belonging to voters, which it then offered to its own customers.

Yet recently published documents reveal that FullContact was investigated by Facebook alongside Cambridge Analytica in 2015-16 and asked to remove some Facebook data from its services after allegedly breaking the social network's policies.

Despite this, the company continued to sell other Facebook data to NationBuilder for another three years, allowing NationBuilder's customers to automatically match people's email addresses to their social media accounts for use in political campaigns.

Last week, after an inquiry from the Telegraph, FullContact said it would delete all Facebook data from all of its services to "minimise any risk of confusion", while denying any wrongdoing.