LONDON (Reuters) - British Foreign Secretary Boris Johnson told voters not to dismiss opposition Labour leader Jeremy Corbyn as a harmless "mutton-headed old mugwump" but rather to see him as a serious threat to the economy and national security.
In his first intervention in the campaign ahead of a snap election due on June 8, the Conservative politician argued that it would be disastrous for Britain if veteran leftist Corbyn became prime minister instead of Theresa May.
In a column in The Sun newspaper, Johnson said people who watched Corbyn "floundering" in debates with May in parliament with his "meandering and nonsensical questions" may draw the conclusion that he would never be prime minister.
"Well, they say to themselves: he may be a mutton-headed old mugwump, but he is probably harmless," Johnson wrote.
"Have you felt a pang of sympathy for his plight? If so, fight it."
The Oxford English Dictionary defines "mugwump" as "a person who remains aloof or independent, especially from party politics", although the term also appears in the Harry Potter books with a different meaning. It was unclear exactly what Johnson meant by using the word.
He said that at a time when Britain faced threats from a "revanchist Russia", a "semi-deranged regime" in North Korea, and the Islamic State group, as well as the challenge of delivering Brexit, only May was capable of leading the nation.
There was no immediate comment from Corbyn, who after decades on the fringes of Labour unexpectedly became party leader in 2015 on a wave of grassroots enthusiasm but has clashed with Labour's members of parliament.
Labour's spokeswoman on foreign affairs, Emily Thornberry, called Johnson's comments "crass and offensive".
"It seems Boris Johnson has finally been allowed out of hiding, on the condition he only talks delusional nonsense," she said in a statement.
There had previously been much speculation in the media about what role Johnson would play in the election campaign.
He was the most prominent campaigner for Britain to leave the European Union ahead of last year's referendum, and was widely seen as a leading contender to be prime minister, although when the position became vacant after the referendum he decided not to run.
The main Conservative message so far in the campaign has been that the nation needed May's "strong and stable leadership" and that Corbyn would bring chaos.
Opinion polls suggest the Conservatives enjoy a huge lead over Labour, leading some political commentators to criticize the ruling party's campaign strategy as pointless.
(Reporting by Estelle Shirbon; editing by Guy Faulconbridge)