Builder and amateur treasure hunter Laurence Egerton has uncovered 20,000 Roman coins from the 4th century in England. Courtesy Laurence Egerton

AN amateur treasure hunter found the biggest ever haul of 4th century Roman coins — and slept in his car for three nights to guard them.

Laurence Egerton, 51, uncovered the coins — dating from AD260 to AD348 — in East Devon, England, in November last year.

The semiretired builder took up metal detecting seven years ago, and was stunned at his luck.

“Between finding the hoard and the archaeologists excavating the site, I slept in my car alongside it for three nights to guard it,” Mr Egerton told English newspaper, The Telegraph.

The Seaton discovery is one of the largest Roman coin hoards ever found. The combined weight is 68kg! #PASmillionth pic.twitter.com/njzYOsB3KZ — British Museum (@britishmuseum) September 26, 2014

He said he initially found two small coins the size of a thumbnail near the top of the soil in a field close to the previously excavated site of a Roman villa.

His continued to dig after his metal detector indicated there was more iron in the ground.

That’s when Mr Egerton found his treasure.

“The next shovel was full of coins — they just spilt out over the field,” he told The Telegraph.

“It’s by far the biggest find I’ve ever had. It really doesn’t get any better than this.

“I’m fascinated by history although I was never really interested at school. Over the years I have found lots of interesting items but never anything of this magnitude.”

This coin, called a nummus, was struck in AD 332 and shows a personification of Constantinopolis #PASmillionth pic.twitter.com/rvX7UaT1iv — British Museum (@britishmuseum) September 26, 2014

The #PASmillionth find is a copper alloy coin, part of a hoard of 22,000 Roman coins found in Seaton, Devon pic.twitter.com/6h50uyG6n8 — British Museum (@britishmuseum) September 26, 2014

Although the coins represented only a few months’ wages for a Roman soldier back in the day, historians say they are now worth tens of thousands of dollars.

The collection, now known as the Seaton Down Hoard, are on temporary display at the British Museum.

The Royal Albert Memorial Museum & Art Gallery in Exeter hopes to raise money to buy the collection and has appealed for public donations, the Independent reported.

Finder Laurence Egerton: 'It’s by far the biggest find I’ve had — it doesn’t get any better than this!' #PASmillionth pic.twitter.com/muhkJNgJ3U — British Museum (@britishmuseum) September 26, 2014

Mr Egerton will reportedly be eligible to split the money from the sale 50/50 with the landowner, but said he’d like to keep one coin as a memento.