Sources say company has run seemingly independent pro-Brexit campaigns since 2017.

A series of hugely influential Facebook advertising campaigns that appear to be separate grassroots movements for a no-deal Brexit are secretly overseen by employees of Sir Lynton Crosby’s lobbying company and a former adviser to Boris Johnson, documents seen by the Guardian reveal.

Investigation follows revelations of Lynton Crosby firm running ad campaign calling voters to lobby MPs.

It is the government’s responsibility to defend against election interference and tackle the broken market that lets it flourish. Failure to do so is a dereliction of its democratic duty.

Russia's influence reaches deep into the British establishment and successive UK governments have turned a blind eye to it, lawmakers were warned, according to multiple sources familiar with testimony given to a parliamentary inquiry.