When the then prime minister, Harold Wilson, called Birmingham’s lord mayor, George Barrow, from the top of the a new 177-metre-high tower on 8 October 1965, he was marking a significant leap forward in British telecommunications technology. The call, made during the building’s opening ceremony, was the culmination of four years of tricky construction. Meant to support a boom in long-distance communication, the tower could handle up to 150,000 telephone conversations and 40 television channels at its launch. But the 3,000-tonne structure, which cost £2.5m to build at the time – equivalent to more than £60m today – was also part of a changing London skyline, and was easily the tallest building in the city until the 1980s. Originally commissioned by the General Post Office (GPO), it was designed by staff at the Ministry of Public Building and Works, including Eric Bedford, who had become the ministry’s youngest ever chief architect at 41. It was a once-in-a-lifetime assignment, requiring soaring height to secure line of sight for radio links and transmissions. The sky-grazing tower inspired GPO staff, who were offered a £10 prize to name the new building. But despite offering up submissions including Pointer, Spindle, Liaise, and Telebeacon, the firm eventually settled on an obvious name: Post Office Tower. Within three weeks of its grand opening, more than 50,000 people had visited its observation gallery, helping the gift shop sell more than 1,500 plastic towers. Also popular were its Top of the Tower restaurant and cocktail bar, with a menu that boasted “strawberries, whether in season or not”. Managed by Billy Butlin – owner of the eponymous chain of UK holiday camps – the 34th-floor rotating restaurant gave diners 360-degree views of the city, taking about 23 minutes to complete a full cycle. But terror struck in on Halloween night in 1971, when a bomb exploded in a 33rd floor men’s toilet. The device, planted in the lowest level of the public viewing galleries, blew out thick walls and damaged cars and buildings with falling debris 350 metres (1,200ft) away. A call to police soon after the blast claimed the “Kilburn battalion” of the IRA had been behind the devastating attack, although another caller suggested it had been the work of the Angry Brigade, a group of far-left anarchists. While no one was injured, the damage took two years to repair and the attack resulted in security being ramped up at public buildings throughout the capital. It also precipitated the closure of the gallery and restaurant, the latter of which became invitation-only until it shut for good in 1980 after the owner’s lease expired. By that point, more than 4.5 million people had visited the site, which was renamed the British Telecom Tower in 1984 when the GPO’s telecommunications arm was privatised, and the BT Tower in 1992 when that company rebranded. While the landmark has featured as a key location in television and film – including Doctor Who, the 1967 Peter Cook and Dudley Moore picture Bedazzled and the 2005 film V for Vendetta – it technically remained a state secret for years. Taking or owning photos of the tower was a breach of the Official Secrets Act and the tower was omitted from Ordnance Survey maps until the mid 1990s. Its significance was formally recognised in 2001 when it was declared a national monument by English Heritage, and two years later when it was given Grade II-listed status. And in a symbol of its move into the digital age, in October 2009 an “information band” video screen was wrapped around the 36th and 37th floors. It would go on to feature messages for the London 2012 Olympic Games (when the tower was also used as a launching pad for fireworks), as well as showing Queen Elizabeth II’s first ever tweet in 2014. The tower’s future is now in the hands of New York-based hotel operator MCR, which has agreed to buy the site from BT Group for £275m. The upmarket hotel rooms in prospect are likely to be reserved for well-heeled visitors. But MCR has promised to “open the building for everyone to enjoy”, suggesting the once popular site could again draw thousands of visitors.