Junior Home Office minister Michael Wills has left the government in order to campaign on trade and agriculture issues.

Wills resigned from his unpaid job on Friday claiming he wanted a free hand to push for the abolition of the EU's Common Agricultural Policy and promote fair trade with the developing world.

The former minister for the criminal justice system had responsibility for the Home Office's information technology and correspondence system.

As the MP for North Swindon since 1997 he has also served as a minister for education and in the old Lord Chancellor's Department.

A close ally of Gordon Brown, Wills' move is likely to be seen as an indication of the chancellor's thinking on trade ahead of the Doha summit later this year.

Writing in Saturday's Guardian newspaper, Wills said that he would now link up with the Trade Justice Movement in its campaign for a level playing field between the West and the rest of the world.

"Sometimes office imposes constraints on what you can say and more can be achieved across the field from the backbenches," he said.

"But there is another way to contribute - chosen by thousands two weeks ago who mobilised for trade justice - and that is through building popular coalitions for change.

"MPs have a key role to play in this, without the constraints of office."

Wills said international trade would be seen as a key issue of the times.

"When I try to imagine how history might assess this period, I believe that one of the key judgments will be how far, in times of unprecedented prosperity in the developed world, we have been able to help the world's most vulnerable people out of poverty," he wrote.

"At the heart of that challenge lies the injustice of the global trading system, and the key to that is agricultural trade."

Britain's best role in this would be to achieve the abolition of CAP, which Wills claims, which "dumps subsidised goods" on developing markets and "devastates the efforts made by the world's poorest people to climb out of poverty".

"How can this government persuade the British people of our European destiny, as I believe it must, when nearly half the EU budget is spent on making rich farmers and agribusinesses richer, taxing the food of hard-working families and impoverishing the world's poor?" he asked.