Clare Short, seen by many in the churches as an important ally over issues of peacemaking and international development, has said she is to step down as an MP.
Ms Short was seen as a 'forthright campaigner' for international development by Christian aid agencies who lamented her resignation as International Development Secretary in 2003.
She was also instrumental in arranging for a delegation of US church leaders to meet with Tony Blair in the run up to the invasion of Iraq.
Short resigned as International Development Secretary in protest at the Government's handling of Iraq, and has remained on the back-benches ever since.
She has also opposed a replacement for Trident, alongside many churches, and cited Gordon Brown's backing for it as a reason for her resignation.
Writing in the Independent newspaper she said; "I have been thinking long and hard about whether to contest the next election as a Labour candidate and decided that I will not.
"For me it is a big decision. I have given my adult life to the Labour Party as the best way I could see of increasing social justice at home and abroad.
"I was deeply honoured to serve as the Secretary of State for International Development which demonstrates how extra money, clarity of purpose and high morale can lead to excellence in public service, and to work with my officials to establish the new Department for International Development.
"There are many good things that New Labour has done since 1997, mostly things Labour committed itself to before the New Labour coup, but I have reached a stage where I am profoundly ashamed of the Government. Blair's craven support for the extremism of US neoconservative foreign policy has exacerbated the danger of terrorism and the instability and suffering of the Middle East. He has dishonoured the UK, undermined the UN and international law and helped to make the world a more dangerous place. The erosion of the rule of law and civil liberties has weakened our democracy and increased Muslim alienation.
"Gordon Brown's commitment to a replacement of Trident, in one throwaway sentence, is an insult to democracy. The approach of New Labour to public sector reform has demeaned the precious value of public services. And in addition to the arrogance and lack of principle of New Labour, there is an incredible incompetence. Policy is announced from Number 10 to grab media attention and nothing is properly thought through."
"With the support of only 22 per cent of the electorate, we see power more concentrated in a Number 10 that consults no one, engages in deceit over matters of profound importance and is not held to account by Cabinet, parliamentary party or the wider party. The Prime Minister's powers of patronage turn too many MPs into obedient ciphers who await the call to ministerial office or quiet elders who await the House of Lords."