The Guardian's bias in reporting has long been established over its 200 year history.

See: https://www.historic-newspapers.co.uk/blog/history-of-the-guardian/

"Political Stance 

On the political spectrum, the publication generally supports the mid liberal to left wing side of British political opinion, officially supporting the Liberal Democrats in 2010 for the first time. In 2005 and 2015, The Guardian backed the Labour Party. As of 2017, The Guardian was considered Britain’s most left-wing newspaper by a YouGov poll, with the Mirror not far behind. The poll revealed that 16% of Britons saw The Guardian as “very left-wing,” placing it ahead of other newspapers."

Media Bias Fact Check analysis supports the view of liberal bias, only not to the same degree as expressed in     https://www.historic-newspapers.co.uk/blog/history-of-the-guardian/                          

See the MBFC article:

 https://mediabiasfactcheck.com/the-guardian/