The inquiry report, published today (Thursday) said suicide bomber Salman Abedi should have been identified as a threat on the night of the attack by those in charge of security.
Allerton High School pupil Sorrell Leczkowski , who lived in Adel, was one of 22 people killed during the attack which followed an Ariana Grande concert at the arena on May 22, 2017.
Neil Hudgell, of Hudgells Solicitors, who represents the bereaved families Sorrell, said: "This inquiry has strongly demonstrated that there was an inexcusable catalogue of failings at every level which made the venue an attractive target to a terrorist attack, failed to deter or prevent the outrage, and as a result contributed to the loss of life and injury."
In his 196-page report examining security arrangements at the venue where 22 people were murdered and hundreds were injured at the end of the concert, inquiry chairman Sir John Saunders found there were a number of missed opportunities to prevent or minimise the “devastating impact”.
Sir John said he considered it was likely Abedi would still have detonated his device if confronted “but the loss of life and injury is highly likely to have been less.”
Among the others who were killed were Courtney Boyle, 19, a Leeds Beckett University student from Gateshead; Kelly Brewster, 32, from Sheffield; Wendy Fawell, 50, from Otley; and Angelika and Marcin Klis, a couple from York.
Mr Hudgell added: "Significantly, at the time, despite the country's national threat level for a terrorist attack being classed as severe, the Government did not have laws in place to enforce venues such as the Manchester Arena and other concert venues to take appropriate counter-terrorism measures in such an environment.
"As a result of these combined failings, thousands of young people who attended the concert on that night were left an open and vulnerable target for terrorists because the security around the venue and event was nowhere near what it needed to be. There were gaps and failings galore.
"If the shortcomings in the UK legal and regulatory framework were not already apparent from earlier attacks in this country, the wave of attacks across Europe including the Charlie Hebdo, Stade de France and Bataclan attacks in 2015 should have left no doubt that existing provisions were inadequate.
"What is more alarming is that those systemic failings remain in place now, four years on."
Manchester-born Abedi, 22, of Libyan descent, walked across the City Room foyer towards an exit door and detonated his shrapnel-laden device, packed into his bulging rucksack, at 10.31pm on May 22 just as thousands, including many children, left the concert.
Sir John said: “No-one knows what Salman Abedi would have done had he been confronted before 10.31pm. We know that only one of the 22 killed entered the City Room before 10.14pm. Eleven of those who were killed came from the Arena concourse doors into the City Room after 10.30pm.”
Sir John said: “The security arrangements for the Manchester Arena should have prevented or minimised the devastating impact of the attack. They failed to do so. There were a number of opportunities which were missed leading to this failure.
“Salman Abedi should have been identified on 22nd May 2017 as a threat by those responsible for the security of the Arena and a disruptive intervention undertaken. Had that occurred, I consider it likely that Salman Abedi would still have detonated his device, but the loss of life and injury is highly likely to have been less.”
He said Arena operator SMG, its security provider Showsec and British Transport Police, who patrolled the area adjoining Manchester Victoria rail station, were “principally responsible” for the missed opportunities.
He added: “Across these organisations, there were also failings by individuals who played a part in causing the opportunities to be missed.”
The inquiry heard Abedi made three reconnaissance trips to the venue, adjoining Manchester Victoria railway station, before his fateful last journey and noticed a CCTV blind spot on the raised mezzanine level of the City Room.
Abedi, dressed in black, crouched down upstairs for nearly an hour, occasionally praying, before he walked down to the foyer.
A concerned Christopher Wild, waiting with his partner to pick up her daughter, earlier approached Abedi upstairs and said he asked him what was in his rucksack but he did not reply. When further pressed, Abedi told him he was “waiting for someone” and asked for the time.
Mr Wild thought “nervous” Abedi looked out of place and raised his concerns at about 10.15pm with Showsec steward Mohammed Agha, who was guarding an exit door, but told the inquiry he felt “fobbed off”.
It was another eight minutes before Mr Agha relayed the concerns to colleague Kyle Lawler as the former had no radio to the security control room and did not believe he could leave his post, the inquiry heard.