The ninth annual list, published today, is the definitive benchmarking tool when it comes to evaluating policy and good practice around sexual orientation issues in the workplace.
The Co-operative is the first big brand retailer to ever enter the Top 10, named third on the list. Housing association company Gentoo group, Nottingham Healthcare NHS Trust and the Home Office complete the top five.
As well as Employer of the Year, the index also recognised the Most Improved Employer, which was Nationwide building society. Network Group of the Year is financial services firm Goldman Sachs and Individual Champion is chief constable, Alex Marshall from Hampshire Constabulary.
The top 100 list was compiled from 376 submitted entries from British employers. Another notable entry in the list was legal firm, Simmons and Simmons, which secured the highest place to date for a legal employer, at number nine.
Security services organisation, MI5 rose 36 places from last year to number 26 and is joined in the top 100 by the Serious Organised Crime Agency (SOCA) at number 40.
ITV is still the only media employer to ever feature on the top 100 list and Rugby Football League is the only sporting organisation to ever feature in the top 100.
Sally Boyle, head of human capital management (HCM) division in Europe, Middle East and Africa (EMEA) and co-sponsor of the LGBT network at Goldman Sachs, told HR magazine: "Over the past few years we have educated our employees around certain diversity issues and encouraged them to talk openly about their sexuality.
"We offer a series of lectures and discussions and offer help and support to employees and I feel we have a very open company."
Boyle added: "To keep improving and promoting good diversity practice in an organisation it must begin at the top."
Nationwide building society, HR director, Keith Astill, told HR magazine: "We want to attract the most talented people in the UK and to do that we have to be recognised as an employer that promotes a diverse workforce.
"We only need to look at our customers that walk through our doors to realise what a diverse country we live in and we want our employees to match that."
Chief constable, Alex Marshall from Hampshire constabulary, who won Individual Champion said he was "extremely proud" for what his organisation has achieved this year. He said: "Everyone around me has been brilliant and I believe we truly are a service that understands people issues."
Marshall added: "There is still a lot more as a police force we can do. One of the main priorities in 2013 is to stamp out homophobic chants at football grounds. A lot of football clubs have cut down on the amount of police they have to save costs so we are training stewards to develop a tough stance and ensure their voices don't just become white noise."
Ben Summerskill, Stonewall chief executive, said. "Every employer in the top 100 has performed impressively. These results are significant as it's just 10 years since we won vital legal protections for gay people in the workplace.
"Britain's 1.7 million gay staff and 150,000 gay university students can now confidently use the Index to select a welcoming employer, or look further, safe in the knowledge that they are protected from discrimination."
Olly Benzecry, senior managing director of Accenture in the UK and Ireland, added: "We're absolutely delighted to receive this recognition from Stonewall. It is a huge credit to our LGBT network. One reason Accenture is a successful business is because of the individuals and groups who make it a great place for people to work and perform at their best."
The full list of winners will be unveiled tonight at a special ceremony in central London.
HRmagazine.co.uk will be publishing the full top 100 at 5pm tonight.
