The politician Frank Field, Lord Field of Birkenhead, who has died aged 81, was an authority on the strengths and shortcomings of the British welfare state.

Labour for all but the end of his time in the Commons, he had many admirers across the political spectrum, including Conservatives who hoped he might join them. The Daily Mail so applauded his socially conservative views that it called him “Saint Frank”.

His experience as a welfare minister in the Blair government was unfulfilling. But in 2010, despite his Labour affiliation, he was appointed the Liberal-Conservative coalition government’s “poverty tsar”.

The source of his expertise was a decade as director of the Child Poverty Action Group (CPAG), from 1969. There, he used the phrase “poverty trap” to describe how a recipient of benefits lost them as their earning increased and remained “trapped” in poverty. This theme fired his lifelong campaign against means testing.

A skilled media operator, he embarrassed the then Labour government by leaking a report that showed poverty had increased on its watch. In 1976 he released cabinet papers that showed that Labour was preparing to ditch plans for child benefit. His causes trumped party loyalty.

Nevertheless, in 1979 he was elected Labour MP for Birkenhead, just as the Tories came into power under Margaret Thatcher. The Trotskyite group Militant had penetrated the local party and regularly tried to deselect him in the 1980s. He made no concessions, refusing in 1981 to vote for Tony Benn as deputy leader and in 1982 calling for Michael Foot’s resignation as party leader.

Opponents managed to deselect him before the 1992 general election, but he warned that he was prepared to stand as an independent and few doubted he would win, probably with an enhanced majority. The national executive committee overturned the decision and he won the re-run contest. In 2009, some Labour MPs suggested he should lose the whip after he had called for Gordon Brown to resign as party leader. Again the maverick, Field threatened he would stand as an independent and saw off critics.

As an MP, Field attacked the spread of means testing and tax credits, arguing that they increased the opportunities for fraud and cheating, and removed incentives to save. He favoured an insurance-based system and believed working people should be compelled to save for a second private retirement pension. He warned that without radical reform, there would be moral and economic disaster.

Welfare accounted for more than a third of public spending but was failing to help those most in need, adding to problems of dependency and, he forecast, would soon run out of money.

His chance to reform the system came when the newly elected Labour prime minister Tony Blair made him minister for welfare reform in the Department of Social Security in 1997. It was an unhappy experience. Field’s relationship with Harriet Harman, the secretary of state, was dysfunctional; the two hardly spoke. She was in cabinet, but he was in charge of welfare, a privy counsellor, on the key cabinet committee and believed he would be made secretary of state in an imminent reshuffle. A further complication was that Brown regarded welfare benefits and taxes as part of his Treasury remit. Field thought Brown made the DSS redundant and soon regretted accepting a post as number two.

He had been appointed to “think the unthinkable”, but Blair and Brown dismissed his suggested reforms because of their high transition costs, believing that voters would see them as extra taxes. He refused Blair’s offer of another post; Harman also left. Blair’s later assessment was that some MPs were not made for office.

Field’s independence, apparent self-perception as being bigger than the party and willingness to make common cause with Conservatives alienated some Labour colleagues – but not his Birkenhead constituents. He admired Thatcher, and paid a late-night visit to her shortly before the second leadership ballot in 1990 to warn she should prepare for defeat and resign with dignity.

It was a surprise that he put his name forward for Commons speaker in 2010, but less of one that he withdrew because of insufficient support; his lack of tribalism had cost him. The downside of Field’s expertise on welfare and his moral approach to issues was that he could appear self-righteous to those who disagreed with him.

Pale and slim, he looked like a young monk even in his 70s. He poured out a steady stream of essays and pamphlets on civil society, welfare and poverty, often for non-Labour thinktanks. But he was more than a policy wonk about welfare and poverty. As a eurosceptic he opposed British entry to the ERM (exchange rate mechanism) in 1990, and was vindicated when Britain was forced to withdraw after two years.

In 2006, long before Ukip’s rise, he controversially attacked large-scale immigration from eastern Europe for its impact on local communities and on job opportunities for unskilled workers. He feared that if mainstream parties did not tackle it, then extreme parties would find a hearing.

He angered the left by favouring curbs on abortion, the retention of nuclear weapons and, for many years, an incomes policy. He was an early advocate of ending the union block vote and of introducing one member, one vote. He annoyed the right by calling for an end to tax relief for mortgages and private pensions. In 2008 he advocated an English parliament and privately warned Ed Miliband early in his leadership that he would have to tackle a looming “English question”.

Born in London, Field came from a Tory-voting, working-class home. His father, Walter, was a labourer in Battersea, south-west London, and his mother, Annie, a teaching assistant. As an adolescent he joined the Young Conservatives before switching to Labour because of its clearer opposition to apartheid in South Africa. After attending St Clement Danes grammar school, then in Hammersmith, west London, he studied economics at Hull University, graduating in 1964, and became a further education teacher in Southwark and Hammersmith. He served as a councillor in Hounslow (1964-68) and unsuccessfully contested the South Buckinghamshire seat in the 1966 general election.

His breakthrough as a campaigner came in 1969 when he became director of the CPAG, building it into a high-profile pressure group. From 1974 he simultaneously headed the Low Pay Unit. This experience was later recognised with his appointment in 1987 as chair of the select committee on social services (later social security) as well as in 2010 the coalition government’s “poverty tsar”. Iain Duncan Smith, the reforming welfare minister in that government, shared many of his views, and was taken aback when Field attacked the coalition’s universal credit and bedroom tax.

In 2015, having persuaded the then Conservative home secretary Theresa May to introduce the Modern Slavery Act, he was asked by her to suggest the contents of the bill, and he chaired the ad hoc committee to scrutinise it. That year, too, he was elected chair of the work and pensions select committee.

He continued his individualist stand in his last years. In 2018 he resigned the Labour whip for what he regarded as the party’s poor handling of antisemitism charges and was then deselected by his local party. Not wanting to go quietly, although already unwell with cancer, he founded a Birkenhead Social Justice party to fight the 2019 general election.

Labour feared for the seat and poured in party workers from outside. He came second to the Momentum-backed Labour candidate. The following year he was appointed a crossbench life peer.

During a debate in the Lords on assisted dying in 2021 his friend Lady (Molly) Meacher read out a letter from him announcing that he was dying and now supported a bill to legalise assisted dying for the terminally ill.

Field was a high church Anglican in the tradition of Archbishop William Temple – in his book Saints and Heroes (2010) he expressed his admiration for Temple – a member of the Church synod and an early advocate of the ordination of women.

Early last year Field published a memoir, Poverty, Politics and Belief.

He served on the board of Cool Earth, Feeding Britain, and the Frank Field Education Trust. In 2022 he was made a Companion of Honour.

He is survived by two brothers.

• Frank Field, politician, born 16 July 1942; died 23 April 2024

• This article was amended on 25 April 2024. Frank Field died on 23 April, and not 23 March.