So went they [Lancaster and his friends] together into the North; and with them they had 700 men of arms, and come to Boroughbridge.

And when Sir Andrew of Harcla ‑ that was in the North Country through ordinance of the King, for to keep the country of Scotland [in check]‑ heard tell how that Thomas of Lancaster was discomfited, and his company were discomfited at Burton upon Trent, he ordained him a strong power, and Sir Simon Ward also, that was sheriff of York, & come & meet the barons at Boroughbridge; & anon they broke the bridge, that was made of tree.  And when Sir Thomas of Lancaster heard that Sir Andrew of Harcla had brought with him such a power, he was sore afraid, & sent for Sir Andrew of Harcla, & with him spake ......

There might men see archers drawn up on both sides; and knights also fought together wonder[fully] sore; and among others, Sir Humphrey de Bohun, Earl of Hereford, a worthy knight of renown throughout all Christendom, stood and fought with his enemies upon the bridge.  And as the noble lord stood and fought upon the bridge, a thief, a worthless creature, skulked under the bridge, and fiercely with a spear smote the noble knight into the fundament, so that his bowels came out there.  Alas the sorrow! for there was slain the flower of solace and of comfort, & also of courtesy.  And Sir Roger of Clifford, a noble knight, stood ever, and fought, & well and nobly him[self] defended as a worthy baron; but at the last he was sore wounded in the head, & Sir William of Sulley & Sir Roger of Bernesfeld were slain in that battle.  And Sir Andrew of Harcla saw Sir Thomas of Lancaster's men left and slain.  Anon he & his company came to the gentle knight, Thomas of Lancaster, & said 'Yield, Traitor! Yield!'  The gentle Earl answered so, and said: 'Nay, lords! Traitor be we none, and to you will we never us yield while our lives last; but believe we have to be slain in our truth, than yield ourselves to you'.  And Sir Andrew again cried upon Sir Thomas's company, yelling as a wolf, and said: 'Yield you, traitor taken! Yield you!' and with a loud voice said: 'Beware sirs, that no man of you be so hardy, upon life and limb, to mishandle Thomas of Lancaster's body'.  And with that word, the good Earl Thomas went into a chapel, and said, kneeling down upon his knees, and turning his face towards the Cross: 'Almighty God!  To you I yield myself, and put myself into thy mercy'.  And with that, the base worthless creatures leapt about him, on every side that Gentle Earl ... and despoiled him of his armour, & clothed him in a robe of Ray, that was his squire's livery, and forth led him unto York by water.

Brie, The Brut : The chronicles of England, 1906 i 217‑220.

The medieval English has been modernized here. The long exchange between Lancaster and Harcla before the action began is omitted here.