Foreign Secretary William Hague has said he was "deeply saddened and disappointed" by Uganda's decision to extend the ban on homosexuality.
The bill, which was signed into law by President Yoweri Museveni, calls for first-time offenders to be sentenced to 14 years in prison and makes it a criminal offence not to report someone for being gay.
He said: "The UK strongly opposes all discrimination on any grounds. We question the Bill's compatibility with Uganda's constitution and international treaty obligations.
"There can be no doubt that this Bill will increase persecution and discrimination of Ugandans, as well as damage Uganda's reputation internationally."
Homosexuality is already a criminal act in Uganda and the new bill is set to recommend a term of life imprisonment for those found guilty of "aggravated homosexuality", defined as repeated gay sex between consenting adults.
Before signing in the anti-gay laws, Ugandan President Yoweri Museveni said that he was privy to scientific "facts" saying: "No study has shown you can be homosexual by nature."
Ugandan President Yoweri Museveni has said "you can come back with worms" after engaging in gay sexual acts as he claimed the nation's anti-gay legislation that will see repeat 'offenders' locked up for life is backed by scientific "facts".
Mr Museveni, a devout evangelical Christian, said homosexuals were "heterosexual people" who act as "prostitutes because of money" in an astonishing statement deliberately intended as a rebuke of "Western countries".
He said he had received scientific "facts" before signing in the anti-gay laws, saying: "No study has shown you can be homosexual by nature."
He said homosexuality in Uganda has been "provoked by the arrogant and careless Western groups that are coming in our schools and recruiting homosexuals into homosexuality and lesbianism".
Mr Museveni also issued a sharp warning to outside nations not to interfere on the matter.
"I would advise Western countries, this is a no-go area," he said. "I don't mind being in a collision course with the West. I am prepared."
In a statement by Desmond and Leah Tutu, the pair said they were "disheartened" by Ugandan President Yoweri Museveni's decision to sign an anti-gay bill.
The statement reads: "When President Museveni and I spoke last month, he gave his word that he would not let the Anti-Homosexuality Bill become law in Uganda. I was therefore very disheartened to hear last week that President Museveni was re-considering his position...
"We must be entirely clear about this: The history of people is littered with attempts to legislate against love or marriage across class, caste and race. But there is no scientific basis or genetic rationale for love. There is only the grace of God.
The statement added: "There is no scientific justification for prejudice and discrimination, ever. And nor is there any moral justification. Nazi Germany and apartheid South Africa, among others, attest to these facts."
Ugandan President Yoweri Museveni has signed into law an anti-gay bill that toughens already strict legislation against homosexuals.
The move defies warnings from the United States that relations could be complicated by the new rules.
Museveni's signature will please a staunchly conservative local constituency that is vehemently opposed to homosexuality, but risks alienating Western aid donors.
The new bill punishes anyone convicted of having gay sex with jail terms up to life, according to a draft of the legislation.
It also makes it a crime to fail to report someone for breaking the new law, again according to drafts.
Museveni signed the bill during a press conference at State House in Entebbe, close to the capital Kampala.
Pope Francis said that gay people should not be judged or marginalised and should be integrated into society.Read the full story ›
While admitting that the Pope's change in tone about gay priests is a "small step" that "does matter," Richard Lane from gay rights group Stonewall told ITV News there was much more to be done.
A next step would be a "strong statement against the harassment and persecution of gay people around the world [from the Pope]," Mr Lane said.
"He's got an unprecedented position at the Church with over a billion people so for him to make that statement would be incredibly significant."
Gay rights group Stonewall told ITV News it is baffled by the Pope's comments on gay priests.
While many lesbian, gay and bisexual Catholics will no doubt welcome this change in tone, the Pope’s criticism of those who lobby for gay equality sounds baffling when his Church still lobbies ferociously worldwide against gay people’s basic human rights.
Stonewall had objected to a section of the Pope's comments which had followed headline remarks about not wanting to judge priests who are gay:
"The problem is not having this [homosexual] orientation. We must be brothers. The problem is lobbying by this orientation, or lobbies of greedy people, political lobbies, Masonic lobbies, so many lobbies. This is the worse problem," he had said.
Pope Francis addressed journalists in a remarkably relaxed manner at a press conference aboard the Papal Aircraft.
He tackled the sensitive issues of gay priests, women priests and the problems of paedophilia within the Roman Catholic Church.
ITV News Correspondent Paul Davies reports:
Pope Francis reached out to gay people in his first news conference, which was held aboard the papal plane as he returned to Rome from his historic maiden trip overseas to Brazil.
Pope Francis told reporters he would not judge priests for their sexual orientation: "If someone is gay and he searches for the Lord and has good will, who am I to judge?"
The pontiff's words signal a move away from the views traditionally held by the Roman Catholic Church - his predecessor, Pope Benedict XVI, authored a document that said men with deep-rooted homosexual tendencies should not be priests.
During his 90-minute news conference the pope appeared relaxed and smiled frequently as he answered every question.
The pontiff even thanked the journalist who raised allegations reported by an Italian newsmagazine that one of his trusted monsignors was involved in a scandalous gay tryst.
Francis said he investigated and found nothing to back up the allegations.
Once he had returned to Rome, Francis tweeted to his 2.7 million followers: "I am back home, and I assure you that my joy is much greater than my exhaustion!"