08 May, 2017
But this very personal attack on Corbyn, in a column for the UK's Sun tabloid newspaper, was the Conservative politician's first direct foray into campaigning for the June 8 snap general election and came after United Kingdom political commentators had noted his low profile.
Damascus denied any involvement in the chemical attack, and the Syrian army said it did not have chemical weapons as they were destroyed under a 2013 deal, monitored by the Organization for the Prohibition of Chemical Weapons (OPCW).
Corbyn also said he would consider ending airstrikes against IS in Syria and push for all parties to return to the negotiating table in a bid to end the civil war.
Liberal Democrats declined to discuss Ms Johnson's move.
He also dismissed suggestions this week from David Cameron that trade talks with the European Union would only begin after the divorce bill had been settled.
In a speech on Wednesday - Mr Johnson's first intervention in the general election campaign - he warned at the London Mayor's Banquet in the City of London that "some plaster may fall off the ceiling" in the Brexit negotiations.
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.
"Well, they say to themselves: he may be a mutton-headed old mugwump, but he is probably harmless", Johnson wrote. I've never been involved in that and never will be.
"We approach this in a responsible, serious way".
Mr Corbyn promised a country that is "properly housed", as he unveiled research showing Labour councils have built an average of almost 1,000 more homes since 2010 than their Tory counterparts.
Opinion polls suggest the Conservatives enjoy a huge lead over Labour and that May's personal popularity ratings are much higher than Corbyn's.
"The mood I'm picking up on the streets is very, very different".
'But when the BBC called it for the take-control freaks, as dawn spread over sea and the super-yachts squatting in the harbour, I went down to the beach and, by the shores of the Med, I sat down and wept'.
