The most empirical and robust study regarding bias was performed by Cardiff University in 2013. Its major finding regarded the dominance of Conservative party political sources in BBC coverage; in coverage of immigration, the EU and religion, they accounted for 49.4% of all source appearances in 2007 and 54.8% in 2012.
The data also showed that the Conservative Party received significantly more airtime than the Labour Party. In 2012, Conservative leader and then Prime Minister David Cameron outnumbered Labour leader Ed Miliband in appearances by a factor of nearly four to one (53 to 15), and governing Conservative cabinet members and ministers outnumbered their Labour counterparts by more than four to one (67 to 15).
In reporting of the EU the dominance was even more pronounced with party political sources accounting for 65% of source appearances in 2007 and 79.2% in 2012.
In strand two (reporting of all topics) Conservative politicians were featured more than 50% more often than Labour ones (24 vs 15) across the two time periods on the BBC News at Six
This is evident right up to the 2019 election - BBC Question Time editing out audience laughter at Prime Minister Boris Johnson's fumbling responses, and soft-shoeing his ascendancy by excusing him from the tender mercies of Andrew Neil - unlike his opposition.
https://theconversation.com/hard-evidence-how-biased-is-the-...