Claire O’Neill, who was stripped of her role as president of the COP26 conference taking place in November, said a rift behind the scenes between the Scottish and UK Governments had put the event at risk.
The outspoken attack by the former clean growth minister threatens to overshadow the official launch of the summit today, attended by Mr Johnson and broadcaster David Attenborough.
In a letter to Mr Johnson published by the Financial Times, Ms O'Neill told him: "You promised to 'lead from the front' and asked me what was needed: 'Money, people, just tell us!' Sadly these promises are not close to being met."
She added: "This isn't a pretty place to be and we owe the world a lot better."
She said the Prime Minister had not convened the Cabinet subcommittee on climate change that he had promised, adding that the Government was "miles off track" in setting a positive agenda for the COP26 summit in Glasgow in November, and that promises of action "are not close to being met".
In an interview with the BBC’s Today programme, Ms O’Neill claimed Mr Johnson had admitted he didn’t understand the issue of climate change.
"The Prime Minister has made incredibly warm statements about this over the years," she said. "He has also admitted to me that he doesn't really understand it. He doesn't really get it, I think is what he said."
In her letter to Mr Johnson, Ms O'Neill also suggested his personal animosity towards Scotland's First Minister Nicola Sturgeon is endangering the summit and claimed the Prime Minister is considering relocating the event to an "English location" because of "ballooning costs".
She told the BBC she urged Mr Johnson to give the First Minister a role at COP26, but the recommendation was dismissed by the Prime Minister "with salty language". Ms O'Neill also claimed that the Scottish Government has behaved "disgracefully".
A Scottish Government source rejected claims of a rift, saying discussions over the conference were “all very good natured” and that there was no suggestion of the venue being changed. The Scottish Government will be represented at today's launch.
However, amid concern over the cost to Police Scotland of protecting the event, Scottish Police Federation Calum Steele posted on twitter: “This standoff has been known about in police circles for some time. Another obstacle the police service has to navigate in amongst planning for the largest political conference on earth this year.”
Ms O'Neill stood down as a Tory MP at the general election, and was sacked as president of the talks by the Prime Minister’s special adviser Dominic Cummings on Friday, with the government saying the post would be a ministerial role in future.
She is reportedly consulting lawyers about what she claims were "false, distorted and defamatory" briefings about her record, saying Number 10 was "rumoured" to be behind the briefings.
Ms O'Neill added to the BBC: "My advice to anybody to whom Boris is making promises - whether it is voters, world leaders, ministers, employees or indeed to family members - is to get it in writing, get a lawyer to look at it and make sure the money is in the bank."
The Prime Minister will outline new climate measures at today’s launch, including a ban on sales of new petrol and diesel vehicles being brought forward to 2035.In an official statement, a Scottish Government spokesperson said a replacement for Claire O'Neill was required as soon as possible. They said: "Scotland looks forward to welcoming the United Nations delegates and participants from around the world later this year.
“The Scottish Government has, from the moment COP26 was confirmed as coming to Glasgow, worked constructively with Glasgow City Council, Police Scotland, the UK Government and other partners to secure the safe and successful delivery of the event and we welcome the significant increase in engagement by the UK Government since the beginning of the year.
“Discussions are of course taking place in relation to costs, as with any major event, and Scottish Ministers expect that all costs associated with COP26 will be borne by the UK Government. This includes funding for police as well as the fire and ambulance services to both prepare for and deliver a safe, secure and successful event.
“COP26 is hugely important for tackling the climate crisis and that must be the focus. A new President should be appointed immediately to make real progress in building the relationships that will be needed within the UK, and around the world, to deliver a successful outcome from the conference.”