WILLIAM HAGUE looks incredulous when I ask if it will be a sad moment when he shuts the door on his Commons office for the last time on March 30, the day Parliament is dissolved.

“Sad?” he laughs, betraying what seems like relief that his political career is all but over. “No, it’s my choice, I’m ready for the change. It’s 20 years since I first joined the Cabinet, 1995, and that’s what has really made me decide. And I’m looking forward to doing other things.”

Those other things include writing — he has two heavyweight political biographies to his name — and music. He took up the piano in his 40s. But he has invited The Sunday Telegraph to the Leader of the House’s office to talk specifically about the illegal trade in wildlife products.

This, together with fighting against sexual violence in war, is the cause to which he will dedicate his post-Parliamentary career after the Duke of Cambridge persuaded him to become his envoy in the battle against poaching.

For a lifelong politician who led the Conservative Party into a general election in 2001, conservation is a departure, but Mr Hague, 53, speaks with the passion of a convert.

“Poaching of rhinos has gone up 9,000 per cent since 2007,” he says.

“So it’s a wholly recent and man-made problem that can be conquered. It’s an issue I care passionately about.” Mr Hague has agreed to be the chairman of a task force on the illegal wildlife trade for the Duke’s umbrella organisation United for Wildlife, which brings together conservation groups from all over the world to share ideas and put pressure on governments and industry.

Eight years ago, 13 rhinos were killed by poachers in South Africa.

Last year, the figure was 1,215, with demand for rhino horn rocketing among Asia’s increasingly prosperous middle classes. They are guaranteed a steady supply from organised crime gangs.

In central Africa, terrorist groups such as the Lords Resistance Army, Al Shabaab and Boko Haram are among the poachers killing 35,000 elephants a year for tusks that can sell for tens of thousands of pounds each.

But Mr Hague has no doubt the trend can be reversed if demand is eliminated. He cites the example of shark fin soup, once a delicacy in China but now dropping off the menu due to a high-profile public awareness campaign.

Beijing remains reluctant to direct its considerable power at stamping out the ivory trade. The Chinese government imposed a one-year ban on ivory imports days before the Duke of Cambridge visited the country this month; its effect has been to push up ivory prices by 20 per cent, which is good news for poachers.

“That’s the problem with a one-year ban,” says Mr Hague. “If there is one thing the Chinese and other governments could do now, it would be to impose a complete ban on the trade in ivory and rhino horn.”

In the meantime, Mr Hague is using his considerable clout, and that of the Duke, to persuade airlines, shipping companies and border agencies to cut off the poachers’ trade routes.