By Tom Midlane @GoldenLatrine
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Topless and kitted out in tight red Speedos, Reg looks a world away from his infamous East End criminal persona as he poses for a holiday snap with his bride-to-be.
Taken in southern Spain in November 1964, the ultra-rare colour shots show the couple in happier times.
Another pic shows a relaxed-looking Reg and Frances taking a turn around the bustling streets, with Reggie in an open shirt and sandals.
The shots, which are owned by a private collector, came to light after being put up for auction by Frances' estate in 2014, along with personal letters, diaries and reportedly audio of Frances singing a less-than-complimentary song about her gangster husband.
A number of other lots were bought by the producers of Legend, the new Krays biopic due out in September, with Hollywood heartthrob Tom Hardy set to play both twins.
The seaside snaps are set to be published as part of a new book, The Krays: Read All About It, compiled by Krays experts Steve Wraith, Neil Jackson and Wayne Lear.
Wraith, 43, a close associate of the Krays who acted as Charlie Kray's bodybuilder at Ron's funeral and visited Reg more than 50 times in prison, said: "The photos reveal a completely different side to Reg Kray.
"We're used to seeing the twins as these black and white, steely-eyed macho men, all suited-and-booted, but we now see Reg Kray in a pair of Crocs! I never thought I'd see the day.
"In 1965 when Reg and Frances got married, there's lots of photographs of them in Athens - but if you compare those photographs he's got a white shirt and a black tie.
"He doesn't look like he's on his honeymoon, and Frances looks pained, she doesn't look like she's enjoying it.
"They also paint a different picture of Reg and Frances' relationship to the generally told version - that they were unhappy, that it was never consummated, that Reg was the dominant partner.
"Although this was only a brief second in time, the photos show a relaxed couple, who seem to be happy in each others company, as if they've had a nice holiday."
Frances killed herself in 1967 by taking a fatal drug overdose at the age of 23, amid rumours of depression at Reggie's increasingly erratic behaviour.
And according to Wraith, Frances' family were never happy with the match, and tried to persuade her not to marry Reggie - to no avail.
"The Shea family always frowned on the relationship, especially because Frances was so young, but Reg's reputation went before him," he said.
"He courted her, and there was obviously that early honeymoon phase - he would treat her with the best things because he had money to burn.
"Holidays abroad were easy for the Krays as at this stage they were making a lot from their criminal actities. If she wanted a dress, she got a dress - the very best. If she wanted a pair of shoes, she got that.
"He looked after her, but God help anyone who chatted her up, or knocked her drink - those people would nine times out of ten end up on the floor horizontal.
"Reg was not working a nine-to-five job and that left Frances to wile away the hours inside. And she obviously suffered from depression."
There have also been suggestions that the marriage remembered unconsummated - while Ron was openly bisexual, Reggie never openly commented on claims he was also gay.
Wraith said: "None of us know whether that was consummated, or why she killed herself, but we know they loved each other enough to get married.
"Whether Reg was actually interested in Frances sexually, we'll never know, but these photos definitely show that when they were alone, he certainly put her on a pedestal and he certainly loved her.
"During our visits he often talked about Frances and how he loved her and missed her. She was a lot younger than him, and may well have seen her as a trophy bride, but I think if anything he probably loved her too much."
Despite the twins closeness, Wraith believes that Reggie regularly used holidays as a chance to have a break from his mentally unstable sibling.
He said: "I think sometimes Reg wanted to get away from the madness of Ron, to get some breathing space."
Wraith also revealed that the couple had a holiday to Ibiza booked when Frances passed away.
But despite Reggie's fearsome reputation - he was handed a life sentence in 1969 for the murder of Jack 'The Hat' McVitie - Wraith is adamant that the hardman had another side to him.
He said: "There was definitely a softer side to Reg. As time went on, he certainly mellowed. There's no question, Ron had a mental illness, he was a paranoid schizophrenic.
"Although Ron and Reg were identical twins but Ron suffered a bad bout of diphtheria as a child, they were separated, and I think that was the turning point.
"There wasn't the same understanding of mental illness as there is now - he was a guinea pig, and it obviously had a long term effect.
"Reggie was obviously very influenced by his brother. When Ron was in prison for three years, that was possibly the most successful time in the Krays' business career.
"Reg and his brother Charlie worked together, they set up the Double R club, one of their most successful establishments, but when Ron came out of prison it started turning over a loss.
"Then came the murders and violence. I think there's no doubt that had Ron been locked away earlier, there's an argument that Reg would have been very different. I know from experience he definitely had a caring side, a lot of friends on the outside, and did a lot for charity."
But once married, relations between the pair quickly began to go awry - and never really recovered.
Wraith said: "Things had changed massively by the mid-60s, the police changed their attitude and were investigating their frauds and protection rackets and keeping an eye on their day to day activities.
"I think Frances had decided to listen to her parents - that maybe being with Reg wasn't a great thing and they broke off on more than one occasion.
"I believe Reg went ahead against the Shea family wishes and booked the church.
"I own half the wedding photos - there's one of the two families and it's so miserable. It doesn't look like a happy wedding photo - and they were almost doomed from then on.
"From there they went to Athens on their honeymoon for a week in April, and Reg went out drinking most nights and leaving Frances in the hotel.
"From then on it was a downward spiral. It was a rushed wedding, the families were against it, and Frances had been having second thoughts.
"Shortly after they got married she went off to Ibiza and Torremelinos by herself, Reggie just accompanied her to the airport. That tells you a lot about her mental state.
"Ultimately Reg's behaviour became more and more unpredictable - he was drunk a lot, weapons were found around the house, he was coming back late from nightclubs.
"She just couldn't cope. She was always going to be seen as Reggie's girl, so people stayed away. They were two people with issues on a collision course.
"I think it all became too much for her. We know Reg was heavily addicted to drugs at the time, it's possible the drugs found in the room were prescribed legally to him, it may have just been a tragic accident, but she had been seeing a psychiatrist, there was no note."
The Krays: Read All About It will be published as a limited edition and can be pre-ordered from www.thekrays.net