File:Attributed arms of the Kingdom of Middlesex.svg

Attributed arms of the Kingdom of Middlesex (the "Middle Saxons"), later a part of the Kingdom of Essex (the "East Saxons") in the Anglo-Saxon w:Heptarchy: Gules, three seaxes barwise in pale argent blades uppermost pommelled and hilted or. In 1910 adopted (with difference of a Saxon crown in chief) as the arms of Middlesex County Council. On the existence of a short-lived independent Kingdom of Middlesex, see (re-ordered) text from wikipedia:

• The name reflects the situation of these people being in the middle between the South Saxons, the East Saxons and the West Saxons, and distinguishing them from the Angles in the north. Unlike these neighbours, the Middle Saxons did not manage to create a lasting kingdom of their own. F.M. Stenton (1971) comments that the Middle Saxon's "original independence is at least probable". But according to G. F. Bosworth (1913), 'there is no evidence that Middlesex was originally a separate kingdom, and we may say with a considerable amount of certainty that it formed part of the kingdom of Essex (...).' The area was part of the Kingdom of Essex at the beginning of the 7th century, but was ceded to Mercia in the 9th century (825)
  
  

Text from https://london.wikia.org/wiki/Coat_of_Arms_of_Middlesex_County_Council :

• Coats of arms were invented in the Middle Ages for the Kingdoms of the Anglo-Saxon Heptarchy. That assigned to the Kingdom of the Middle and East Saxons depicted three "seaxes" or short notched swords on a red background. The seaxe was a weapon carried by Anglo-Saxon warriors, and the term "Saxon" may be derived from the word. These arms became associated with the two counties that approximated to the kingdom: Middlesex and Essex. County authorities, militia and volunteer regiments associated with both counties used the attributed arms. In 1910 it was noted that the county councils of Essex and Middlesex and the Sheriff's Office of the County of London were all using the same arms. Middlesex County Council decided to apply for a formal grant of arms from the College of Arms, with the addition of an heraldic "difference" to the attributed arms. Colonel Otley Parry, a Justice of the Peace for Middlesex and author of a book on military badges, was asked to devise an addition to the shield. The chosen addition was a "Saxon Crown", derived from the portrait of King Athelstan on a silver penny of his reign, stated to be the earliest form of crown associated with any English sovereign. The grant of arms was made by letters patent dated 7 November 1910. The undifferenced arms of the Kingdom were eventually granted to Essex County Council in 1932

Further reading:

• "Armorial bearings of Middlesex", The Times. London. 7 November 1910.
• The Book of Public Arms, A.C. Fox-Davies, 2nd edition, London, 1915
• Civic Heraldry of England and Wales, W.C. Scott-Giles, 2nd edition, London, 1953
• C W Scott-Giles, Royal and Kindred Emblems, Civic Heraldry of England and Wales, 2nd edition, London, 1953, p.11