Uganda's anti-gay laws will lead to crimes against humanity comparable to those seen in "Nazi Germany and apartheid South Africa" according to former Archbishop Desmond Tutu.

His comments come as Ugandan President Yoweri Museveni is due to sign the anti-gay bill into law, after previously promising that he would not do so without further scientific consultation.

"Unless I have got confirmation from scientists that [homosexuality] is not genetic, but a behaviour that is acquired, I will not sign the bill," he has said previously.

The bill decrees that homosexual "touching" is punishable by up to life in prison.

Promoting homosexuality, or in any way aiding and abetting others to commit same-sex acts, will be punishable by between five and seven years in jail.

"We must be entirely clear about this," Archbishop Tutu was quoted as saying in the Guardian.  "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. 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."

Elaborating on comparisons with South Africa, Archbishop Tutu said: "In South Africa, apartheid police used to rush into bedrooms where whites were suspected of making love to blacks. They would feel if the bed sheets were warm, crucial evidence to be used in the criminal case to follow. It was demeaning to those whose 'crime' was to love each other, it was demeaning to the policemen – and it was a blot on our entire society."

The bill has also been opposed by US President Barack Obama, who was quoted in the Independent as describing it as a "step backward for all Ugandans".

The US has warned that if the anti-gay bill becomes law it could "complicate" Uganda's relationship with the US, potentially putting £240 million in annual aid in jeopardy.

Sweden, along with several other European countries, had also previously threatened to withhold aid in response to an earlier version of the bill which included a potential death sentence if people were found to have committed acts of "aggravated homosexuality" that included repeated consensual sex.

The sponsor of the bill, MP David Bahati, claimed on the BBC that homosexuality was a "behaviour that can be learned and can be unlearned".

"Homosexuality is just bad behaviour, that should not be allowed in our society."

Archbishop Tutu called on the Ugandan President to change direction: "My plea to President Museveni is to use his country's debate around the Anti-Homosexuality Bill as a catalyst to further strengthen the culture of human rights and justice in Uganda."

Ugandan gay activists speaking to the BBC have said they are "very scared" about the new bill.

"I didn't even go to work today [Monday]. I'm locked up in the house," one activist said.

"And I don't know what's going to happen now. I'm talking to all my activists on the phone. And it's the same, they are all locked up in their houses. They can't move out. They are watching their back to see what happens."

The bill was due to be signed at today.

Out of Africa's 55 countries, 36 criminalise homosexual activity.