The sword, which is believed to date from about 1550, was put up for sale by its former owner Hugh Meller on the internet auction site eBay because his wife could not find anywhere to put it in their house.

But when it was spotted on the internet by members of the Naseby Battlelfield Project group, they realised the weapon, which was made about 100 years before the battle, could reveal soldiers at Naseby were forced to use out-of-date weapons to defend themselves.

The sword was originally found by Mr Meller's father, Charles, on the Naseby battlefield site in Northamptonshire.

When Mr Meller realised it was due to return to Naseby he cancelled the eBay auction and gave it to the historians.

Historian Martin Marix Evans, who has written books on Naseby, said: "It's very exciting. It seems likely to me that granddad's sword was simply prised off the wall and pressed into service because nothing else was available.

"Even though it would have been 100 years old at the time, it seems very likely it was used in the battle, after all it would still have been a lethal weapon."

After investigating the weapon Mr Marix Evans said it shed new light on how the battle was fought, suggesting soldiers solved the problem of depleted armouries by grabbing whatever weapons they could for the fight.

He said: "There'd been no serious production of weapons for about 50 years before Naseby and one of the big problems was getting hold of arms, so they would have just taken anything they could lay their hands on.

"And while we can't say for sure this sword was actually wielded at Naseby, the fact it was found there really implies someone just took it off the wall and went straight into battle."

The sword was inspected by experts from the Royal Armouries as well as historians who found a near identical weapon on the wreck of the Mary Rose.

They said because the sword's tip was missing it was likely it was used in battle and thrown down after it broke.

The sword will be one of the first items to be put on display at the Naseby visitor centre, which members of the Naseby Battlelfield Project hope to see built in the next few years.