The plate was inscribed with the words of the Ulster Solemn League and Covenant in 1912 and was used to print copies of the key historical document, which was signed by hundreds of thousands of unionists pledging to resist Home Rule on the island of Ireland.

The plate was made by Belfast-based printing business William Strain & Sons, a small family-run company founded in 1859, and has remained in the family, passing through the generations, ever since although the printing business no longer exists.

Belfast-based Bloomfield Auctions managing director, Karl Bloomfield, said the plate was extremely significant: “We believe this may be the original head proof plate of the covenant because there are no signs of ink, paper or wear of any kind.”

He explained: “This Covenant document was signed by hundreds and thousands of men and women, with some signing in blood. It was so important.

"Last year also marked 100 years since the formation of Northern Ireland and so in light of these recent centenary events, it has really brought the importance of Northern Ireland history into focus.”

In 2012 the plate was displayed at Belfast’s Linen Hall Library to mark the centenary of the signing of the covenant.

Bloomfield said the closing amount at auction was unexpected.

“We started the bidding at around £5,000 and I’d expected it may fetch up to £10,000 or £12,000 so £15,000 was quite a surprise. We can’t disclose yet who exactly bought it, but I can say it was a Unionist-focused organisation through an online bid.”