Grieving Wigan woman and other bereaved families urge Prime Minister to say whether he attended Downing Street party
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The correspondence is signed by a Wigan woman whose father's death certificate was being signed on May 20, 2020 - the day of the alleged gathering in the No 10 garden, to which more than 100 people were invited.
Hannah Brady and others who lost loved ones in the pandemic have shared their outrage at the revelations, saying the fact Mr Johnson has refused to say whether he attended such a gathering "adds insult to injury".
Ms Brady, who said her father died after contracting Covid on his way to his job as a key worker in Wigan, described the reported party - in the same location where the group met Mr Johnson last year - as a "flagrant breach" of the Government's rules.
In the letter, she stated: "In September of last year you looked me in the eyes in the Rose Garden of Downing Street and told me you had done everything you could to save him.
"You saw photos of his last days that no one had ever seen before.
"It is now clear that whilst my dad's death certificate was being signed and me and my younger sister were grieving alone, dozens of people were gathered, clutching a bottle they had been invited to bring, in the same place you told me you had done everything you could."
Addressing Mr Johnson directly, Ms Brady told him he can "only imagine the pain, anguish and anger this news has brought to me and those of us who lost a loved one to Covid-19".
She also urged him to apologise for the "additional hurt caused" by his reaction to being questioned by broadcasters about the gathering, saying he had "laughed, smirked and seemed to treat it as one big joke".
The letter added: "It is simply a matter of common decency and respect for not only us or the British people, but the office you hold as Prime Minister of the United Kingdom to tell us all whether you attended this flagrant breach of your Government's own rules.
"My Dad didn't have much in his life, but he had his integrity and a sense of right and wrong that he passed down to me. If you did wrong and attended this party, or knew of it happening, do the right thing and own up to it, before any investigation forces you too, not after."
The campaign group had earlier released a statement saying that, if the Prime Minister attended the alleged gathering, his position is "untenable" and he has "lost all moral authority to lead the country".
Those words were echoed by Susie Crozier, who said the latest in a series of allegations about potentially restriction-breaking gatherings has left bereaved families like hers feeling "constantly kicked in the gut."
The private tutor from Sunderland, whose father Howard Crozier died on March 28 2020 after catching Covid in hospital, said she hopes that, with the latest reported incident, there is "enough public outcry now for Boris Johnson to go, frankly".
She described her father's funeral in April that year, attended by two people, as "one of the bleakest experiences I've ever had".
Speaking to the PA news agency, she accused Mr Johnson of "playing us for mugs" by refusing to say whether he attended the gathering along with his now wife Carrie, despite it being widely reported he was there.
She said: "If he hasn't, you know, the courtesy to resign, someone needs to boot him out."
A mother from Crystal Palace whose 18-year-old daughter died from leukaemia in May 2020 said adhering to restrictions at the time of her child's death made an "incredibly painful situation even harder."
Ruby Fuller died five days before the alleged party and had to say goodbye to her grandparents over video call software Zoom.
Her mother, Emma Jones, said she is speaking up about the issue as she knows her daughter would have been at the Prime Minister's door protesting about it.
Ms Jones told PA: "Ruby asked to be remembered by the motto, 'live kindly, live loudly', so I feel I have to shout about this now... if she were alive, she'd be hammering on No 10's door with a protest banner."
Meanwhile, the son of a Bletchley Park codebreaker, whose funeral was on the same day as the gathering, said he is "totally gobsmacked" by the claims.
Ann Mitchell, who was part of a Second World War codebreaking team at the country house near Milton Keynes, died on May 11 2020, aged 97, after testing positive for Covid-19.
Her 63-year-old son, Andy Mitchell, told PA news only five people were able to attend her funeral, which took place in Edinburgh.
"We didn't complain at the time because we knew that was what the situation was... I expected any moment to be pulled over by the police to say, 'Where are you going? What are you doing?'" Mr Mitchell said.
Of the alleged Downing Street gathering, Mr Mitchell said: "It wasn't just a sort of casual, 'Let's go outside at the end of the day', but to actually send out an invitation from the Prime Minister's office to do that, I'm just stunned."
Mr Mitchell's mother deciphered coded German communications at Bletchley Park after being recruited from university.
He said she worked "for the national good", adding: "She not only did her duty and her service but respected all the strictures that came with it.
"So, when you see the email (from) Downing Street in May after about two months of (a) pandemic, saying you've all been working incredibly hard - she did that for two years, without complaining, without thinking, 'Oh, we'll just go for drinks, to hell with it'."
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