Dave Merritt, whose son was killed in last month's London Bridge attack, said the way the tragedy had been exploited for political ends was "crass and insensitive".

Merritt's 25-year-old son Jack was one of two people killed when a former convict attacked people at a prisoner rehabilitation event that Merritt was helping to run on November 29.

Dave Merritt told Sky News that "instead of seeing a tragedy, Johnson saw an opportunity.

"... They immediately said, 'Oh, this is Labour's fault - they allowed this to happen. They had this early release policy,' and so on."

Johnson, meanwhile, tried to focus voters on the prospect of an uncertain result and a divided parliament, which would endanger his plan to lead Britain out of the European Union on January 31.

All 650 seats in the House of Commons are up for grabs in the election, which is being held more than two years early in a bid to break the political impasse over Brexit.

Opinion polls give the Conservatives a lead over Labour, but all parties are nervous about the verdict of a volatile electorate that is weary after years of wrangling over Brexit.

"Polls can be wrong," Johnson said on Tuesday. "We need to be fighting for every vote."

He accuses Labour of offering more "dither and delay" on Brexit. The opposition party says it will negotiate a new divorce deal with the EU and then give voters a choice between leaving on those terms and remaining in the bloc.

"Forty-eight hours from now, our country can choose between going forward, punching through the current deadlock ... or we can remain stuck in neutral," Johnson said during a visit to a construction equipment factory in central England, where he drove a bulldozer through a plastic foam wall with "Gridlock" written on it.

Merritt's interview was another late hurdle in a campaign that had gone smoothly for Johnson until a newspaper ran a photo of four-year-old Jack Williment-Barr sleeping on the floor of the Leeds General Infirmary as he awaited treatment because no bed was free.

The opposition Labour Party led by Jeremy Corbyn painted the boy's plight as a symptom of Britain's ailing health system, which has suffered under years of Conservative government austerity measures.

A video of the prime minister briefly declining to look at a mobile phone photo of Jack on a journalist's phone - and then placing the phone in his pocket - has been viewed more than 1 million times.

The incident quickly became caught up in a storm of social media claims, counterclaims and conspiracies.

Several prominent journalists, including the political editors of the BBC and ITV, tweeted a claim by anonymous Conservative officials that a party worker had been punched by a protester while Britain's health secretary visited the hospital.

When footage emerged showing that no assault had taken place, they apologised - but a media storm was already raging.

Some social media users circulated claims that the photo of Jack, first published by the Yorkshire Evening Post, was staged. Editor James Mitchinson had to defend it, explaining how the newspaper had verified the story.

Labour found itself embarrassed, meanwhile, by the leak of a phone recording to the right-wing political website Guido Fawkes in which the party's health spokesman suggested that the party would lose Thursday's election because voters "can't stand Corbyn".

Jonathan Ashworth said his unguarded remarks were merely banter with a Conservative friend and claimed he had been trying to "psych him out like football managers do".

"Obviously with the benefit of hindsight I've been too clever by half and I look like an idiot as a result of doing it," Ashworth told the BBC later. "But I thought I was having a private conversation with someone who I've always had conversations with over the years."

Australian Associated Press