Robert de Beaumont, the Earl of Leicester, a supporter of young Henry who had been in Normandy and chief of the aristocratic rebels, takes up the charge next.

He raises an army of Flemish mercenaries and crosses from Normandy back to England to join the other rebel barons there, principally Hugh Bigod, Earl of Norfolk, who is based at the castle of Framlingham.

He lands at Walton in Suffolk in late September or early October.

After some inconclusive fighting, Leicester decides to lead his men to his own base of Leicester, but royalist forces prevent this.

The earl's base there had recently come under attack by royal forces and thus needs reinforcement, but another reason for the movement may have been friction between de Beaumont and Bigod and Bigod's wife, Gundreda.

The Battle of Fornham is fought on October 17, 1173 between rebel forces under the command of Leicester and royal forces under the command of Richard de Lucy, the Chief Justiciar as well as Humphrey de Bohun Lord High Constable, Reginald de Dunstanville, the Earl of Cornwall, William of Gloucester, the Earl of Gloucester, and William d'Aubigny, the Earl of Arundel.

The rebel forces are numbered at three thousand mercenaries, and the royal forces include at least three hundred knights as well as the Earl of Norfolk's son, Roger Bigod, who has remained loyal to the king.

Along with these knights, the royal forces also have the local levies and the military followings of three earls of Gloucester, Arundel, and Cornwall.

The rebels are caught fording the River Lark near the present towns of Fornham St Genevieve, Fornham All Saints, and Fornham St Martin in Suffolk at a location about four miles (6.4 kilometers) north of Bury St Edmunds.

With his forces split, Leicester's cavalry is captured and his mercenaries are driven into nearby swamps where the local peasants kill most of them.

Leicester is captured, as is his wife, Petronilla de Grandmesnil, who had put on armor herself.

Leicester will remain in captivity until January 1177 when some of his lands will be returned to him.