Brendan Coyle reportedly had a Twitter spat with a fan in which he told her to "go f**k yourself".
The actor - who plays John Bates in Downton Abbey - got into an argument with the fan over his disapproval of Margaret Thatcher.
NHS consultant Roberta Knighton told him that his Twitter comments about the former prime minister were "beneath" him.
However, the actor replied: "Hard to know how to respond to that... but go f**k yourself feels about right."
He added: "You have no idea what is beneath me... p.s. go f**k yourself x". He then blocked her from viewing his account, according to The Sun.
Knighton said: "He comes across as utterly sweet and gentle then speaks to people like that. I was shocked."
Brendan Coyle said last year that the cast of Downton Abbey know when the show will come to an end.
