The regulatory body carried out an investigation following a Channel 4 new report in June which raised concerns about calls made by Blue Telecoms, a firm based in Neath, South Wales, on behalf of the Conservative Party.

The investigation found that two sections of the written scripts used by those making the calls on behalf of the Conservatives in the lead up to the 2017 general election “crossed the line from legitimate market research to unlawful direct marketing”.

The ICO has stopped short of formal regulatory action, because “the overall campaign was market research”, head of enforcement Steve Eckersley wrote in a blogpost.

As part of the investigation, the regulator was concerned by two sections referencing Theresa May and Jeremy Corbyn in relation to policy choices.

Eckersley said: “We’ve stopped short of formal regulatory action because the overall campaign was genuine market research. The two sections we had concerns about were not enough to trigger formal enforcement action when considered along with the campaign as a whole.

“But we have been clear about what we expect in the future. We’ve warned the party that its campaigns must be rigorously checked for questions that fall outside the bounds of market research.

“While we did not resort to the full force of our regulatory powers in this case, we will continue to keep an eye on all political parties in the run up to future elections and repeat our advice to them that they must comply with data protection and privacy laws when campaigning.”