This is a very important article from Declassified UK. This extract gives you the gist of how the secret services in collusion with the press, in particular The Times, The Telegraph, The Daily Mail and the Sun pulled all stops to prevent Corbyn from winning both the 2017 and 2019 elections, and it had no connection with facts:
The stories — which quote former or current members of the army, navy and special forces, as well as MI5, MI6 and an ex-senior civil servant — have averaged one every six weeks since Jeremy Corbyn was elected leader of the Labour Party in September 2015. There have, however, been significant spikes in frequency during the 2017 and 2019 general election campaigns.
There is a strong suggestion that, for some stories, intelligence officials have themselves provided secret documents to journalists as part of what appears to be a campaign.
Every story has been picked up across national print media, often setting the news agenda and chiming with statements from Conservative government ministers. Nearly every story appeared in four papers — The Daily Telegraph, The Times, the Daily Mail, or The Sun.