A man has been convicted of cashing in a fake National Lottery ticket to claim a £2.5m jackpot.

Edward Putman conspired with a Camelot insider to cheat the system and present a counterfeit slip to claim the outstanding top prize in 2009.

He devised the plan with his friend Giles Knibbs – who then worked in the securities department at the lottery operator – with the pair submitting a deliberately damaged forgery just before the 180-day limit to stake claims expired.

The fake National Lottery ticket used by Putman. Photograph: Crown Prosecution Service/PA

But the fraud unravelled after Knibbs confessed to friends that he had “conned” the lottery before killing himself after an angry row about how the winnings were divided.

Jurors at St Albans crown court found Putman guilty of fraud by false representation on Friday after a two-week trial. Putman did not appear to make any reaction to the verdict.

The genuine winning ticket, which was bought in Worcester, has never been discovered.

Putman, a convicted rapist, was paid the jackpot by Camelot despite the bottom part of the mangled slip missing the barcode, the trial heard.

Jurors were told the scam began to fall apart after the friendship between former business partners Knibbs and Putman deteriorated.

Knibbs’s behaviour became increasingly erratic and he began revealing details of the fraud to friends after failing to receive what he said was his agreed £1m share of the prize. He confronted Putman in a heated argument in June 2015, breaking Putman’s car wing mirrors and stealing his phone.

The lottery worker was subsequently arrested for burglary, blackmail and criminal damage after Putman complained to police. He later killed himself after fearing he would go down for 10 to 15 years for blackmail, the trial heard.

Evidence suggested Knibbs was initially paid £280,000 by Putman for his part in the ruse, followed by much smaller increments totalling £50,000.

The scam began after Knibbs saw documents being printed containing details of big wins that had not yet been claimed while working late one night.

Prosecutor James Keeley told the trial there was some trial and error in producing a successful forged ticket, with several different specimens made, each with one of the 100 different possible unique codes on the bottom.

Knibbs had claimed Putman went to 29 different shops as the clock ticked down to claim the cash, providing a different ticket at each, before the right number was found. Keeley said Putman eventually submitted the correct code at a shop in High Wycombe, on 28 August 2009.

Despite his multimillion-pound windfall, three years later in 2012 Putman was sentenced to nine months for benefit fraud after going on to claim £13,000 in housing and income support.

His previous convictions also include the rape of a teenager in the early 1990s, for which he was sentenced to seven years.

In 2016, the Gambling Commission fined Camelot £3m for breaching its operating licence regarding controlling databases, investigating prize claims and paying out prizes.

The Crown Prosecution Service said it would “take steps to recover his fraudulently acquired winnings”.

District crown prosecutor Tapashi Nadarajah said: “Edward Putman deceived the National Lottery operators with his ‘winning’ ticket, making him a millionaire; but his lies unravelled with the tragic death of his co-conspirator who he wasn’t prepared to share the money with.

“We used accounts from Knibbs’s friends, as well as documented evidence on his phone and financial transactions, to build a compelling case against Putman. This was further strengthened by indisputable evidence provided by an expert in the scientific examination of questioned documents.

“They found significant differences between the printing on genuine tickets and that on Putman’s ticket, concluding his ticket was not genuine.”

Putman is due to be sentenced by the judge, Philip Grey.