British journalism has never been held in high esteem by the public, though the BBC and other broadcasters have always been more trusted than the tabloids, and for good reason. In recent years, however, levels of trust in the BBC have fallen across the political spectrum, and particularly on the left. The Reuters Institute, shows the percentage of those identifying with the left who mostly trust the BBC dropped from 75% in 2019 to 60% in 2020.

In the BBC’s annual plan last year it acknowledged that its reputation for impartiality had been weakened, and said that new editorial guidelines were being developed to try to restore trust. Alongside these new rules on impartiality, which were published in October, is new guidance on the use of social media, which has been a particular focus of recent debates around BBC bias.

Social media platforms have been blamed for increasing the perception of political partisanship by giving rise to online “echo chambers”, and the BBC itself has flagged up “social media vitriol and political polarisation”. However, criticisms have also been made of BBC journalists’ use of social media, particularly Twitter, which as a medium for breaking stories, relaying anonymous briefings and airing political gossip is not subject to the usual editorial controls. Addressing this has been a priority for the new director general, Tim Davie, who promised MPs in September that he would take “hard action” against anyone breaching impartiality rules on the site.

As is often the case with questions of BBC impartiality, though, there is much more heat than light. At present, thanks to some activist newspapers and Conservative MPs, the debate seems to centre on Gary Lineker’s use of Twitter to occasionally express liberal views on Brexit and immigration, rather than on the BBC’s journalism.

There is now a large body of scholarship on the influence of social media on journalism, and a number of recent studies examining journalists’ follow and interaction networks on Twitter. In the first quantitative study to look at BBC journalism specifically, two Aston colleagues, Killian Mullan and Gary Fooks, and I examine the use of the platform by 90 BBC journalists tweeting in their official capacity, using data extracted in early 2019.

Rather than looking at particular journalists or specific tweets – the meaning of which will always be contested – we examine the Westminster MPs followed, retweeted or mentioned by these journalists in aggregate. Our data doesn’t tell us anything about the personal politics of BBC journalists, but it does show which political actors are generally thought to be significant, allowing us to examine the culture of BBC news in a relatively objective way.

Follows and Twitter interactions not only fall under the BBC’s new social media rules but were covered in the previous guidance, which already stipulated that “we should ensure that we reflect due impartiality in our choice of accounts to follow”.

Given this policy, we would expect Twitter followings and interactions to be broadly even between the political parties. In fact, we find some striking differences. Putting aside the single Green MP, Caroline Lucas, the MPs with the highest average BBC following among BBC journalists were the now disbanded breakaway group Change UK, followed by the Liberal Democrats.

Just as striking as the attention enjoyed by centrist politicians, though, is the relative marginality of MPs in parties exclusively representing constituencies outside England. While Change UK MPs had an average of 11.5 BBC journalists as followers, and the Lib Dems 6.9, the equivalent figures for the SNP and Plaid Cymru were 2.7 and 2 respectively.

The two main political parties at Westminster were broadly similar in terms of their Twitter following among BBC journalists, but if we dig a little deeper, some notable differences emerge. Despite the fact that Labour MPs have more followers on Twitter, BBC journalists are more likely to follow Conservative MPs in general, and high-profile Conservative MPs in particular; and additionally are more likely to follow members of the cabinet than members of the shadow cabinet.

It could be argued that this pattern reflects the BBC’s editorial remit to “hold power to account” – meaning that the key decision-makers attract more attention. But this would be consistent with following members of the shadow cabinet at least as much as government ministers, since this is precisely the constitutional role of the official opposition.

Within the two major political parties, we also looked at party factions. In the case of the Conservative party we found that the “moderate” MPs who broke with Boris Johnson’s government to vote for an extension to Brexit in October 2019 were more followed by BBC journalists than party loyalists. This may reflect the potential of these MPs to obstruct the government and thereby to generate news stories – but it is also provides further evidence of the orientation of BBC journalists towards centrist politicians.

Within the Labour party we categorised MPs according to a leaked document, reportedly drafted in January 2016, that identifies five groups of Labour MPs on the basis of their perceived “hostility” to the then party leader, Jeremy Corbyn. Labour MPs in the group most hostile to Corbyn, and the second most hostile group, each had more BBC journalist followers on average than any other group, including the “core group” of Corbyn-supporting MPs, many of whom were in the shadow cabinet and were among the most followed Labour MPs on Twitter.

These findings challenge the conventional wisdom about the BBC as either a politically impartial or left-leaning organisation. The evidence seems to suggest that the BBC leans to the centre right. Our findings on the Labour party factions confirm claims by Corbyn supporters that the former leader’s critics in the party were given inordinate attention by the BBC. This is underscored by the fact that Corbyn himself – who had by far the largest Twitter following of any MP – was followed by fewer BBC journalists than either his predecessor, Ed Miliband, then deputy leader Tom Watson, or Change UK’s leading light, Chuka Umunna.

The marginalisation of parties in Scotland, Wales and Northern Ireland are equally concerning, given our increasingly devolved politics, not to mention the BBC’s commitment to “take account of the different political cultures and structures in different parts of the UK” and “ensure that differing main strands of argument in nations, regions and communities receive due weight and prominence”.

If the new director general is serious about restoring trust in the organisation’s news and current affairs, it must address the central problem: according to the social scientific evidence, there is not a leftwing bias, but a political culture that is overly focused on the ruling party and the leading players in Westminster, and too often neglectful of outside perspectives.

• Tom Mills is lecturer in sociology at Aston University. He is the author of The BBC: Myth of a Public Service