Writing in the Financial Times, Browne said: "During my years in business, I came to realise that anything that fosters an inclusive environment makes good business sense.
"People are happier and more productive and make more money for their company when they feel they are included and can be themselves. Giving gay couples the freedom to marry sends an important signal of inclusiveness. If that freedom helps gay people to be themselves in both their private and professional lives, it will eliminate one more barrier to a true corporate meritocracy."
Browne quit BP in 2007 after losing a battle to suppress newspaper reports about his relationship with another man. He said the decision to protect his secret was born out of fear. "If I had seen gay men in loving, stable and legally recognised public relationships of the sort my parents were in, I would have found it easier to come out. Civil partnerships perform this function up to a point, but they are not equal to marriage."
On Monday, the Lords will discuss a "wrecking amendment" seeking to derail the gay marriage bill, which was passed in the Commons in May despite the opposition of 133 Conservative MPs.
Ex-West Midlands chief constable Lord Dear has tabled the amendment to refuse the bill a second reading, which would in effect kill it.
Some 86 peers are due to speak in Monday's debate with a final vote on Tuesday. The bill is expected to face a tough passage through the Lords and Dear claims the vote is "too close to call".
Browne, who as a peer will be in the chamber, said he would support it for practical business reasons – because marriage has evolved and should follow society in the growing acceptance of homosexuality – and because of his personal experience.
New York mayor Michael Bloomberg also expressed his support for gay marriage, saying it is "consistent with democracy's promise of equal rights for all people".
Bloomberg wrote in the Guardian: "As long as government is in the business of handing out marriage licences, all couples – regardless of their sexual orientation – deserve equal status in the eyes of the law."
guardian.co.uk © Guardian News and Media Limited 2010
image: © Trodel