Sam Harris, then 11, was hurt when a much taller and heavier 15-year-old caught the left side of his head with a heel while they were somersaulting on the castle in September 2005.

He suffered a skull fracture resulting in a "very serious and traumatic brain injury" and now needs round-the-clock supervision, said his counsel, Susan Rodway QC.

The amount of damages, which are likely to exceed £1 million, will be assessed at a later stage if not agreed.

Sam, who brought the case through his mother, Janet, of Long Lane, Gedney Hill, near Holbeach, claimed Catherine and Timothy Perry, who hired the castle for their triplets' 10th birthday party, were to blame for the accident, which happened on the playing fields behind their home in Rochester, Kent.

But the Perrys denied liability, and blamed Sam's father, David, who was brought into the proceedings as a third party and a third defendant, on the basis of his alleged lack of supervision.

Mr Justice David Steel, at London's High Court, ruled that the Perrys, who were insured, were liable and dismissed the claim against Mr Harris.

He said he found it probable that Mrs Perry did give Sam permission to use the castle, but that the level of supervision was inadequate.

The accident was caused by the shortfall in supervision – because no one was in a position to intervene to stop the somersaulting – and by an older boy being allowed to use the castle at the same time as the younger and smaller children.

The judge gave the Perrys permission to appeal because of the ruling's "potential significance", not just to the couple, but also to those who operate inflatable equipment.

He said that if the appeal was not pursued, Sam should receive an interim payment of £100,000.

After the hearing, Mr and Mrs Harris said in a statement: "Our sole motivation for pursuing this claim was, and has remained throughout, to obtain damages for our son, Sam, who suffered a very serious injury in this accident.

"We are both very relieved that the court has found that Mr and Mrs Perry were negligent as this will now enable us to obtain the specialist support and treatment that Sam needs.

"While we never wanted the case to conclude at trial, we are pleased that this has provided an opportunity to highlight just how dangerous bouncy castles can be if they are not supervised properly.

"We appreciate that thousands of children enjoy playing on bouncy castles every year, and we would not wish to stop that happening, but it is vital that those hiring them supervise them properly if accidents like Sam's are to be avoided.

"We would also like to stress that Sam's mother did not start the case against his father. It was the hirers of the castle, Mr and Mrs Perry, who suggested from the outset that responsibility for this accident lay with Sam's father and not with them.

Customers must sign a hiring agreement
Lisa Benin-Casa, the owner of A&M Castle Hire, in Emneth, near Wisbech, said: "We ask our customers to sign a hiring agreement if they want to use one of our bouncy castles, stating how many children should be allowed on.

"If they refuse to sign the agreement, we would not hire out the bouncy castle.

"Having public liability insurance gives peace of mind and is a back up in case insurance is needed."

The owner of a bouncy castle company near Market Deeping, who did not want to be named, added: "This case was about supervision, which is the responsibility of the person who hires the bouncy castle. That is the advice we give people, to supervise users of the bouncy castle.

"We do have public liability insurance but that wouldn't have made a difference in this case. It seems a shame to blacken the reputation of bouncy castles for what is a one in a five year incident.

"It is terribly sad because the vast majority of people have a whale of a time on bouncy castles."