In second place is Hampshire Constabulary while Ernst & Young came third.
Ernst & Young won the award for Employee Network Group of the Year and Nottinghamshire Healthcare NHS Trust has named Most Improved Employer.
Stonewall's index is based on a range of key indicators that this year included the largest-ever survey of lesbian, gay and bisexual employees, with over 7,000 participants. This consistently revealed that staff satisfaction levels were highest at the top ranking organisations in the index. Gay staff working at the top 25 workplaces reported a satisfaction level almost 10% higher than workplaces outside the Top 100. Staff were also more likely to disclose their sexual orientation in monitoring exercises at the highest-ranking organisations.
Results of the 2010 Stonewall Top Employers list will be announced at a winner's ceremony held tonight in London hosted by international law firm Simmons & Simmons.
Ben Summerskill, Stonewall's chief executive, said: "Competition was fiercer than ever to secure a place on the 2010 Top Employers List. We received more entries than any previous year from employers that understand and have benefited from Stonewall's research which found that gay people are far more likely to buy goods or services from companies they know are gay-friendly. The index is a powerful tool used by Britain's 1.7 million gay employees and 150,000 gay university students to decide where to take their talent and skills."
Brendon Riley, chief executive officer at IBM UK & Ireland, added: "IBM is extremely proud to accept this important accolade. In 2009 we celebrated the 25th anniversary of IBM's global LGB non-discrimination policy - but we're constantly looking for new ways to enrich and broaden the scope of what we can achieve. At IBM, we pride ourselves on delivering a working environment that reflects equality of opportunity and experience for all. Diversity constitutes our character, our identity and ultimately our success - it is in our DNA."
