The Conservative party has been accused of becoming a home for “cranks” after some of its candidates at the general election were revealed to have shared conspiracy theories on social media.

The posts seen by the Observer include the suggestion that positive tests for Covid-19 were “mass psychosis at work” and that the Black Lives Matter movement might be an attempt to “bring down British society”.

Labour said some of the postings shared by Tory candidates suggested “serious concerns” about the calibre of the party’s would-be MPs. Some candidates have already deleted social media accounts, or removed posts, or locked access to accounts.

One Tory candidate in London is revealed to have referred to the London mayor Sadiq Khan as a “snivelling little drip” who had done more to damage the capital “than the Luftwaffe”. Another wrote in a book in 2005 that “we must teach people to be prejudiced once again”.