According to a story on the Guardian news website, the provisional findings (the final report will be released in March) indicate that the polls underrepresented a series of key groups, including over-70s, young non-voters, and busy voters.  

The inquiry, commissioned by the British Polling Council (BPC) and the Market Research Society (MRS), was led by Patrick Sturgis, a professor of research methodology at Southampton University. 

“The emerging upshot is that the companies are going to have to be more imaginative and proactive in making contact with – and giving additional weight to – those sorts of respondents that they failed to reach in adequate numbers in 2015,” Sturgis was quoted as saying. 

Jane Frost, chief executive of MRS, said: "We would like to offer our thanks to Prof. Patrick Sturgis and his panel for the speed of their response and commitment to producing robust and insightful conclusions.  We look forward to receiving their final report in March and acting on its recommendations.”

"Today’s unveiling will provide the polling companies and everyone else with an interest in its work to hear and respond to the Inquiry’s initial conclusions," said Prof. John Curtice, president of the British Polling Council. "It hopes that this process will prove helpful to the work of the Inquiry and ensure that its final conclusions are based on the strongest possible body of evidence.’

The findings from the preliminary report are to be presented at an event this afternoon (January 19 ) at the Royal Statistical Society.