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Southend will be granted city status as a tribute to Sir David Amess, the Tory MP who led a decades-long campaign for the Essex seaside town until his death.
The Prime Minister addressed MPs in the Commons on Monday and said the decision had the backing of The Queen.
Mr Johnson said Sir David was a "seasoned campaigner of verve and grit" who "never once witnessed any achievement by any resident of Southend that could not somehow be cited in his bid to secure city status for that distinguished town".
MPs cheered as the Prime Minister announced: "I am happy to announce that Her Majesty has agreed that Southend will be accorded the city status it so clearly deserves."
Boris Johnson said all MPs mourn with Sir David's family, adding in the Commons: "Sir David was taken from us in a contemptible act of violence striking at the core of what it is to be a member of this House, and violating both the sanctity of the church in which he was killed and the constituency surgery that is so essential to our representative democracy.
"But we will not allow the manner of Sir David's death in any way to detract from his accomplishments as a politician or as a human being.
"Sir David was a patriot who believed passionately in this country, in its people and in its future. He was also one of the nicest, kindest and most gentle individuals ever to grace these benches."
"That Sir David spent almost 40 years in this House, but not one day in ministerial office, tells everything about where his priorities lay."
Sir David worked closely with the local council on his city campaign and never missed a chance to mention it at Westminster.
"It had almost become a bit of a joke in the House of Commons," said Councillor Alex Bright from Southend-on-Sea Borough Council.
"As soon as he'd stand up to mention it there was always a groan in the House of Commons because of how many times they heard Sir David banging on about it!" he added.
Southend was one of several towns competing for city status as part of the Queen’s platinum jubilee celebrations in 2022.
Sir David was often seen in Westminster and on the campaign trail sporting 'Make Southend a city' merchandise, including slogan-adorned baseball caps and face coverings.
Sir David's campaign to push for city recognition has spanned more than two decades.
Ian Gilbert, leader of Southend-on-Sea Borough Council, said he felt a "mixture of emotions" following Monday's announcement.
He said: "It is clearly what Sir David would have wanted, that has been made clear by his family and his close colleagues over the past couple of days.
"While I don't want it to have come in these circumstances, I'm still pleased and proud that it is happening.
"It is something that Sir David was extremely passionate about. Members of Parliament will tell you that he worked on the claim for Southend to be given city status almost every time he spoke in the House. This was something that he cared about.
"I'm proud of the council and the community and very humbled and thankful for Sir David's work to bring this about and I hope that after the difficulties that we have gone through, this will bring people together."
Leader of the Southend Conservative group, Councillor Tony Cox, said: "To hear the announcement today that Southend-on-Sea will be granted city status was an emotional but incredible moment.
"Sir David, your legacy will forever live on in Southend-on-Sea.
"I cannot thank Her Majesty the Queen and the Prime Minister enough for granting that legacy, but what truly breaks my heart is that he is not around to see it.
"I am sure he will be looking down on us now saying, 'my work in Southend is now complete'."
Ron Martin, chairman of Southend United Football Club, added: "Sir David worked tirelessly over many, many years for the town to be granted city status, and wonderful that his efforts have been recognised. A truly fitting tribute and lasting legacy."
Local shop worker Lee Jordison described it as "great news". The 40-year-old, who works at Hicks butchers in Leigh-on-Sea, saw the aftermath of the attack at Belfairs Methodist Church.
He said: "I think it's great news and a fitting tribute to a man who campaigned for so long to want to have Southend awarded city status. I'm sure the town of Southend will be equally as happy!
"It's just a real shame he never got to see it happen but I'm sure his family will be proud that this was honoured because of him."