A new study by Ordnance Survey (OS), the Met Office, and 4 Earth Intelligence has identified several UK cities that are expected to experience the most significant warming by the end of this century due to the urban heat island effect. This phenomenon occurs when cities become significantly warmer than their surrounding rural areas, primarily due to human-made materials like concrete and asphalt that absorb and retain heat. The research created a heat vulnerability index for 71 cities across Great Britain, excluding Northern Ireland. Portsmouth currently holds the highest vulnerability rating and is projected to remain the most at risk throughout the century. By 2040, all 71 cities are expected to reach high heat vulnerability, with nearly two-thirds facing the most severe level by the end of the century. The study highlights that urban areas can be up to 10 degrees Celsius hotter than surrounding regions. This is due to the combination of hard surfaces, buildings, and lack of natural features like trees, green spaces, and waterways that help cool the environment. This summer was provisionally the hottest on record for the UK, and the research underscores which cities are currently experiencing extreme urban heat and which are likely to be among the warmest in the future. In Portsmouth, neighborhoods like Somerstown, Portsea, Southsea, and Fratton are particularly vulnerable to urban heat due to limited green spaces and high amounts of hard surfaces. These areas also tend to overlap with regions experiencing higher levels of deprivation. Meanwhile, the City of London and the City of Westminster are identified as the most heat-retentive urban environments in the UK. The City of London has the highest built-environment score and the least effective natural cooling, while Westminster ranks third in current heat vulnerability and second in future projections, behind Portsmouth. The economic impact of extreme heat is significant, as seen in London during the 2022 heatwaves, which were estimated to have cost £1.5 billion, affecting productivity, health, education, energy use, transport, and emergency services. The OS heat vulnerability index combines current temperatures, the structure of urban environments, and climate projections to assign each city a score from 1 to 5, with 3 to 4 indicating high vulnerability and 4 to 5 indicating severe risk. In contrast, Milton Keynes is among the cities best prepared for heat, ranking 69th out of 71. It has the lowest built-environment vulnerability in England and the most effective natural cooling, with more natural surfaces than hard ones. The study aims to help urban planners and emergency services create targeted strategies to improve resilience and prepare for future heat events.