The former Goldman Sachs star brought in to build the Northern Powerhouse just quit government

Former Goldman Sachs economists Lord Jim O'Neill has quit his unpaid role in the Treasury.

Sky's political correspondent Darren McCaffery first reported the resignation, which has been rumoured for a while.

Downing Street's press office confirmed the resignation to Business Insider over email (scroll down to see Lord O'Neill's full resignation letter.)

Lord O'Neill joined the government last May to take charge of the "Northern Powerhouse" initiative, one of former Chancellor George Osborne's pet projects. The "Northern Powerhouse" project aimed to drum up investment in northern cities to create economic hubs that could rival London.

O'Neill famously coined the acronym BRICs to describe the emerging market powerhouse economies of Brazil, Russia, India and China, and served at Goldman Sachs' chief economist.

However, reports emerged last month that Lord O'Neill was unhappy with new Prime Minister Theresa May's approach China, after May personally intervened in plans for a Chinese-funded, new nuclear power plant at Hinkley Point. O'Neill had helped to broker the funding.

Lord O'Neill downplays this friction in his resignation letter, instead painting his departure as a result of the conclusion of a review of anti-microbial resistance which he led.

However, he does say he "primarily joined however for the specific purpose of helping deliver the Northern Powerhouse, and to help boost our economic ties with key growing economies around the world, especially China and India." He pointedly says: "I am pleased that, despite speculation to the contrary, both appear to be commanding your personal attention."

Lord O'Neill says in his letter that he will now move from the Conservative Party benches in the House of Lords to be a cross bench Lord.

Here is Lord O'Neill's resignation letter:

Dear Prime Minister,

I have decided to resign as a Minister in the government.

This morning I returned from New York where I had the pleasure of witnessing the UN high level agreement on antimicrobial resistance (AMR), which is essentially the end goal of the Review that I was asked to lead in the spring of 2014. This part of my role in government has come to its natural end, although of course, I was asked initially to lead the Review, independent of government.

Leading this Review has been one of the most stimulating roles I have ever undertaken and I am very grateful to all those involved, especially my Review team, and all those across government that allowed the UK to take such a global leadership position in this key threat to the world. It goes without saying that our success is symbolic of the way that the UK can be influential in a post BREXIT world.

I joined the government with the AMR Review already well developed in terms of our ideas and influence, but entered office, believing its goals would be enhanced by being a Minister. I primarily joined however for the specific purpose of helping deliver the Northern Powerhouse, and to help boost our economic ties with key growing economies around the world, especially China and India and other rapidly emerging economies.

The case for both to be at the heart of British economic policy is even stronger following the referendum, and I am pleased that, despite speculation to the contrary, both appear to be commanding your personal attention. I am leaving knowing that I can play some role supporting these critical initiatives as a non-governmental person.

I look forward to moving to the cross benches of the Lords, and wish you every success with the exciting challenges and opportunities ahead, and thank you for allowing me the privilege to serve in government.

Yours sincerely,

Jim O'Neill

And here's the response from the Prime Minister:

Dear Jim,

I was sorry to receive your letter of resignation. I would like to thank you for your service to the Government over these last two years, both as Commercial Secretary to the Treasury and as Chair of the Review on Anti-Microbial Resistance.

You have made a significant contribution to driving forward the Government's work on delivering growth beyond the South East through the Northern Powerhouse and on promoting stronger economic links with emerging economies, including China and India. You have laid important foundations in these areas, and the Government will build on them.

I would particularly like to pay tribute to your ground-breaking work on Anti-Microbial Resistance. You should take great pride in seeing your review culminate this week in the UN high level agreement. You have played a vital role in building global consensus on this important issue which will have long-lasting benefits.

I wish you well for the future.

Yours sincerely,

Theresa May