The portrait of Alan Turing, the pioneering computer scientist and British war hero, was made from hundreds of vials of blood, all donated by gay men.

The painting was commissioned by campaign group Freedom to Donate, who unveiled it at an event at a reception at the Houses of Parliament on Monday.

Artist Conor Collins told BuzzFeed News that almost all the blood is from medical professionals who are unable donate blood, because of their sexuality.

"With the exception of one person, all the blood in the painting is from GPs, surgeons, nurses. They're all medical professionals who, because of who they are, can't donate blood," he said.

"One of them was a heart surgeon and you literally trust him to open you up and hold your heart in his hands and yet we apparently, according to the law, don't trust him to donate blood.

"It was surprisingly easy to get the blood. Once I started telling people about the piece and the protest we were making, they were instantly eager to help."

Men who have had sex with another man in the last 12 months can't give blood, according to NHS guidelines.