与此同时，蛮族于408年袭击了不列颠，但这些袭击似乎都被击退。于410年后，西罗马皇帝霍诺留似乎写信命令不列颠各城市需要自我保护。尽管这有时会引起争议。[26][27][28]日耳曼劫掠者从5世纪中叶开始在不列颠东部河谷定居。[29] 其后内战似乎已经爆发，这些内战被后世学者解释为亲罗马和独立聚落之间，或“国教会”和伯拉纠主义党派之间（Myres 1965年，Morris 1965年），农民和地主之间的阶级斗争（Thompson 1977年，Wood 1984年），或是一场城市精英的政变（Snyder 1988年）。Stuart Laycock（英语：Stuart Laycock）在2008年的著作《不列颠尼亚失败的国家Britannia the Failed State》探究一个新近的观点认为不列颠根据各部落本身，猛烈地分裂成一个个不列颠人的王国; 猛烈地分裂是有争议的，但显然大多数不列颠尼亚罗马公社（英语：civitates）逐渐转变为王国。乡郊似乎继续著和以前一样的生活，而欧塞尔的圣日耳曼努斯（英语：Germanus of Auxerre）造访不列颠时描述城镇的生活规模缩小。王权的长期斗争取代了过去罗马行省中央管治的方式。
吉尔达斯（英语：Gildas）认为军阀沃蒂根（英语：Vortigern）召开了一个“议会”以寻找应对蛮族威胁的方法。议会按照罗马的惯例选择雇用撒克逊雇佣兵。一段时间过后，这些雇佣兵调转枪头反对不列颠人并劫掠城镇。据说不列颠人领袖安布罗休斯·奥理安在很长一段时间内在许多战役中与他们作战。巴顿山战役（英语：Battle of Badon）大约在490年这一时期将近结束时发生，后来的资料来源声称这场战役是亚瑟王赢得的，尽管吉尔达斯没有在著作中透露他的身份。此役之后，迎来一段漫长的和平时期。不列颠人似乎已经控制了英格兰和威尔士，边界大致是从约克到般尼茅夫以西一带。日耳曼的撒克逊人控制了从东约克郡到林肯郡，也许还有诺定咸郡，再到东盎格利亚和东南英格兰一带的弧形东部地区。
吉尔达斯大约在540年用拉丁文写成描述了不列颠历史的著作，但前半部分（其他可用资料来源）极度混乱。他严词批评了不列颠西部的五位统治者的罪孽—顿诺尼亚的君士坦丁、奥勒留·卡尼努斯(Aurelius Caninus)、德梅泰（英语：Kingdom of Dyfed）的沃蒂波尔(Vortipor)、库内格拉苏斯(Cuneglasus)和马格洛库努斯(Maglocunus)（或称为Mailcun或后来拼写为格温内斯的迈尔贡（英语：Maelgwn Gwynedd））。在著作中他还攻击了不列颠的教士。他提供有关不列颠的饮食、服饰和娱乐的资讯。他写道，布立吞人被杀、移居或被奴役，但没有给出数字。
在6世纪晚期，撒克逊人再一次出现了扩张时期，从日后统治威塞克斯王国的王族于552年占领威尔特郡的Searoburh (现今的老沙伦)（英语：Old Sarum）为起点，扩张包括在577年迪勒姆战役（英语：Battle of Deorham）后进入中南至西南部的科兹窝地区，虽然对《盎格鲁-撒克逊编年史》中这一时期的条目的准确性被受质疑。但现代作家经常在没有明确证据的情况下，说这些征服将西南英格兰的布立吞人（后来称为西威尔士人）与威尔士的布立吞人分割开。（就在讨论该时期之后，于611 年的车士打战役（英语：Battle of Chester）可能已经将后者与英格兰北部分割开。）但直到570年代，布立吞人仍然控制着英格兰和威尔士大约一半的地区。
Esmonde-Cleary, A. S. (1989) The Ending of Roman Britain. London: Batsford
Fouracre, Paul (ed.) (2005) The New Cambridge Medieval History, Volume I, c.500–c.700. Cambridge: Cambridge University Press
Jones, Michael E. (1996) The End of Roman Britain Ithaca: Cornell University Press
Halsall, Guy (2013) Worlds of Arthur. Facts and Fictions of the Dark Ages. Oxford: Oxford University Press
Higham, Nicholas (1992) Rome, Britain and the Anglo-Saxons. London, Seaby
Higham, Nicholas (1994) The English Conquest: Gildas and Britain in the Fifth Century. Manchester University Press
Jones, Michael (1996) The End of Roman Britain. Ithaca: Cornell University Press
Lapidge, Michael & Dumville, David (1984) Gildas: New Approaches. Woodbridge: Boydell
Morris, John (1973) The Age of Arthur
Morris, John (1980) Nennius: British History and the Welsh Annals. Chichester: Phillimore
Morris, John (gen. ed.) Arthurian Period Sources volumes 1–9, general editor: John Morris, Phillimore & Co, Chichester (includes full text of Gildas & Nennius, St Patrick material and various annals and charters)
Halsall, Guy Barbarian migrations and the Roman West, 376–568 Cambridge University Press; illustrated edition (20 Dec 2007) ISBN978-0-521-43491-1 pp.217–218
Discussion in Martin Millett, The Romanization of Britain, (Cambridge: Cambridge University Press, 1990) and in Philip Bartholomew 'Fifth-Century Facts' Britannia vol. 13, 1982 p. 260
Michael Jones（英语：Michael Jones） andJohn Casey（英语：John Casey）, 'The Gallic Chronicle Restored: A Chronology for the Anglo-Saxon Invasions and the End of Roman Britain', Britannia19, (1988), pp.367–398; R.W. Burgess, 'The Dark Ages Return to Fifth-Century Britain: The 'Restored' Gallic Chronicle Exploded', Britannia21, (1990), pp.185–195
"Town and Country: The End of Roman Britain", World Archaeology12(1), (June 1980:77–92); Simon T. Loseby, "Power and towns in Late Roman Britain and early Anglo-Saxon England" in Gisela Ripoll and Josep M. Gurt, eds., Sedes regiae (ann. 400–800), (Barcelona, 2000: pp. 319–370 (on-line text互联网档案馆的存档，存档日期25 January 2012.) makes a strong case for the discontinuity of urban life.
Philip Barker's excavation in the Baths Basilica at Wroxeter, (1975) is noted by R. Reece, "Town and country: the end of Roman Britain", World Archaeology, 1980.
H.R. Loyn, Anglo-Saxon England and the Norman Conquest, 2nd ed. 1991:15f: "it is altogether unlikely that organized town-life can have survived through the troubles of the fifth and sixth centuries. Gildas lamented the destruction of the twenty-eight cities of Britain, and there is no reason to doubt the essential truth of his statement" (p16).
A.S. Esmonde Cleary, "The Roman to medieval transition" in Britons and Romans: advancing an archaeological agenda. ed. S. James & M. Millett, (York: Council for British Archaeology, 2001)
John Davey, "The Environs of South Cadbury in the Late Antique and Early Medieval Periods" in Debating Late Antiquity in Britain AD 300–700. ed. Rob Collins & James Gerrard, (Oxford: British Archaeological Review, 2004)
Archaeological Resource Assessment of the Isle of Wight: Early Medieval period. Compiled by Ruth Waller, Isle of Wight County Archaeology Service, August 2006 Oxford Archaeology （页面存档备份，存于互联网档案馆）.
Helena Hamerow（英语：Helena Hamerow）, 'The earliest Anglo-Saxon kingdoms' in The New Cambridge Medieval History, I, c.500–c.700. ed. Paul Fouracre (Cambridge: Cambridge University Press, 2005), pg. 265.
Jones, Arnold Hugh Martin, John Robert Martindale, John Morris, The Prosopography of the Later Roman Empire, volume 2, Cambridge University Press, 1992, ISBN0-521-20159-4
Halsall, Guy Barbarian Migrations and the Roman West, pp. 376–568 Cambridge University Press; illustrated edition (20 December 2007); ISBN978-0-521-43491-1, pp. 217–218
Discussion in Martin Millett, The Romanization of Britain, (Cambridge: Cambridge University Press, 1990) and in Philip Bartholomew 'Fifth-Century Facts' Britannia vol. 13, 1982, p. 260
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