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.

Slideshow ( 2 images )

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.

Corbyn accused Johnson of “personal name-calling” and said what was needed was a serious debate about issues such as housing, education, health and foreign affairs.

In a column in The Sun newspaper published on Thursday, Johnson said people who watched Corbyn “floundering” in debates with May in parliament with his “meandering and nonsensical questions” may conclude 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.

“The biggest risk with Jeremy Corbyn is that people just don’t get what a threat he really is,” he said.

Asked in an interview on ITV television what he meant by “mugwump”, which the Oxford English Dictionary defines as “a person who remains aloof or independent, especially from party politics”, Johnson did not explain the word.

“It’s a long time since I read it, but I think it was in ‘Charlie and the Chocolate Factory’,” he said, referring to a popular children’s novel by Roald Dahl.

Asked about the “mugwump” comment while out on the campaign trail, Corbyn dismissed it as a personal attack unworthy of the issues at stake in the election campaign.

“We approach this in a responsible, serious way. I leave that kind of language to others,” he told the BBC.

“We’re eight days into the election campaign and the Tories (Conservatives) have reduced to personal name-calling. I’ve never been involved in that and never will be.”

Corbyn unexpectedly became Labour leader in 2015 on a wave of grassroots enthusiasm after decades on the left-wing fringes of the party. But he has little support from Labour’s members of parliament, many of whom regard him as an electoral liability.

Johnson, one of Britain’s most prominent politicians who was the leading campaigner for Britain to leave the European Union ahead of last year’s referendum, had previously kept a low profile since May called the snap election on April 18.

This prompted much speculation about his positioning and whether May was concerned that the eccentric Johnson may go off-script or upstage her in some way, when the Conservative strategy is to focus on her leadership.

Opinion polls suggest the Conservatives enjoy a huge lead over Labour and that May’s personal popularity ratings are much higher than Corbyn’s.