The London Evening Standard has become the “mouthpiece of the Conservative party” after showing an overwhelming bias towards Zac Goldsmith, the party’s candidate for London’s mayor, in the run-up to next month’s election, according to new research.

The Evening Standard’s Sarah Sands: ‘I’m a journalist with a liking for drama’ Read more

Thirteen out of 15 official press releases from the Goldsmith campaign released during the two months to 12 April were published as news stories in the paper, “reproducing headlines from the news release virtually verbatim”, according to the study produced by the Media Reform Coalition and Goldsmiths University of London.

There were almost twice as many positive headlines about Goldsmith than for his Labour rival Sadiq Khan, who was the subject of twice as many negative headlines. What’s more, Khan’s press releases were far less frequently covered.

The research, published on Wednesday night, stopped well before Tuesday’s front page story trumpeting David Cameron’s view that “Britain will pay the price if Sadiq Khan is elected mayor”.

Justin Schlosberg, chair of the Media Reform Coalition, a network of researchers, activists and campaigners for reform of media policy, said: “The Evening Standard appears to have been operating as the mouthpiece of the Conservative party.” The research suggests that while the features and opinion pages show greater balance, the news stories at the front of the paper does not.

Accusing the Standard of being “systematically biased in the way that headlines and news stories were framed, selected and prioritised”, Schlosberg said: “This is all the more concerning because bias was concentrated in relatively prominent headlines and in news reports where readers are less likely to expect opinion to be mobilised.”

The research found that, although the majority of all stories about the candidates were neutral or balanced, those that exhibited the strongest bias – and especially those that were negative towards Khan – were the most prominent. Negative stories about Khan were more likely to be published on the first ten pages of the paper, with three of them on the front page.

The three biggest negative Khan stories concerned his alleged links to people with extremist backgrounds. Headlines included: “Exposed: Sadiq Khan’s family links to extremist organisation”, and “Minister: Khan is unfit to be mayor”. Khan’s rebuttals were not given due prominence and “would not have been noticed by anyone reading the paper at speed”, according to the research.

In contrast, the two biggest negative stories for Goldsmith related to his tax affairs and were published on page 10. “In both cases the Goldsmith rebuttals were given equal prominence and favourable treatment in editorials,” Schlosberg said.

The total number of headlines in the sample that were positive towards Goldsmith and/or negative to Khan totalled 27 out of 121 articles in total, compared with 14 that were negative towards the Conservative. Articles favouring the Conservative candidate, as opposed to headlines, outnumbered those favouring his opponent by more than three to one and tended to be more prominent than any positive stories about Khan.

Sadiq Khan's concern for London's Jews shames his smearing Tory foes Read more

The Standard denied any bias and said it had given both candidates a platform. The research should have looked at the covered for the entirety of the election, it said, pointing out a recent interview with Khan led the paper and included a spread inside. Khan had made no complaint about coverage, it added.

Goldsmiths’ Prof Angela Phillips said the research showed the need for greater plurality, given the Standard’s dominance with a distributed circulation of more than 900,000 copies per day. “Newspapers are free to campaign for any candidate but where there is only one London-focused newspaper, which is handed out free to commuters across the transport network, editorial bias could have an undue influence on the outcome of the election or deepen distrust in journalism. This research demonstrates the need for a more plural news media,” she said.

In an interview with the Guardian, the Standard’s editor, Sarah Sands, said the paper would be “scrupulous” in ensuring balance in the run-up to the mayoral election on 5 May.

Sands’s predecessor, Veronica Wadley, had been heavily criticised for partisan attacks on the Labour candidate Ken Livingstone, and Sands, a friend of the Conservative incumbent Boris Johnson, supported the Conservatives last time.