As plain Reg Prentice, Lord Prentice of Daventry, who has died aged 77, achieved political notoriety as the highest-ranking Labour figure ever to defect to the Conservative party.
His dramatic switch in 1977, which followed his deselection in Newham North-East after a series of battles with leftwing constituency activists, prefigured a decade of destructive infighting within Labour's ranks, and was all the more shocking because of his background as a trade union official.
But his career was not without solid achievement on the ministerial benches - albeit in both major parties. After defecting, he managed not only to land a safe Tory seat at Daventry but to obtain office under Margaret Thatcher as a junior social security minister, from 1979 until ill health forced his retirement in 1981. Knighted in 1987, the year he stepped down as an MP, he popped up in the 1992 new year's honours list as a peer.
Probably there was something in his background and resilience, rather than in his pugnacious personality, that appealed to the then prime minister, John Major, himself another improbable south London survivor.
Educated at Whitgift school, Croydon, then as now a fee-paying school, Prentice went to the London School of Economics, worked as a temporary civil servant and emerged from military service in the Royal Artillery in the officer class, serving in Italy and Austria during the second world war.
After another spell at the LSE, he graduated and, in 1950, joined the staff of the Transport & General Workers Union (TGWU), then under its formidable leader, Ernie Bevin, Labour's foreign secretary. The union propelled Prentice into parliament as MP for East Ham North in 1957.
A conventional political career beckoned. When Labour finally won back power in 1964, after the internecine 50s, Harold Wilson made Prentice a minister of state, first at Education and Science (1964-66), then Public Buildings and Works (1966-67), and finally in charge of the still-new Ministry of Overseas Development (1967-69).
In opposition, after 1970, Prentice was a red-faced and genial colleague who had a knack of doing well in shadow cabinet elections, sometimes coming top, to the surprise of colleagues. In 1974, with Labour back in power, he entered the cabinet as education secretary, cheerfully helping to abolish direct grant status for leading grammar schools and driving them into the private sector - a problem that haunts Labour governments to this day.
By 1975 Wilson sent him back to Overseas Development. But by this time, troubles were brewing in his renamed constituency, Newham North-East, which he was accused of neglecting, usually an important feature of deselection battles in any party. The larger picture was one of growing economic strain under successive oil shocks, which fed through into rising unemployment and inflation, putting pressure on public services, including councils.
Leftwing militancy among unions was matched by infiltration - or "entryism" in the jargon of the time - by previously banned groups like the Militant Tendency. Prentice had his supporters in Newham, leading councillors included, but he was also opposed by the legitimate left.
What made his battle so memorable was that bedsit Trotskyism was matched in Newham by the arrival of alleged Labour moderates, notably a young Oxford graduate called Julian Lewis, to fight on the MP's behalf. Helped by funding from the Freedom Association (it later emerged), Lewis, now himself the Tory MP for New Forest East and on William Hague's right, briefly won control of the constituency party in late 1976. As befitted a turbulent decade, the language on both sides was extravagant.
When asked to resign, Prentice hit back at the "little gang" who ran the local party, accusing them of "going completely round the bend" and of indulging in a "transparent fiddle". He claimed that the crucial deselection resolution had been tabled without reference to him and that the situation had reached "low farce" and "pure communism".
At Labour's 1976 party conference - the one that saw chancellor Denis Healey recalled from an IMF meeting to face his furious critics in Blackpool - Prentice appealed from the rostrum to have the national executive committee endorsement of his deselection overturned. "Political cowardice as the price of political survival," he called it. The conference was unmoved.
When he joined the Tories, Labour allies, who had backed him, including Shirley Williams, herself to break with Labour four years later, were furious and embarrassed. Naturally, his Newham enemies were gleeful at their early, somewhat self-fullfilling detection of what ex-chief whip Bob Mellis (no leftwinger he) now dubbed "a nauseating traitor". Prentice said at the time that his defection was brought about by "a growing emphasis on class war and Marxist dogma" in the Labour party. Yet he remained loyal to comprehensive schools and to incomes policy even as he became a Thatcher convert.
As Tory-to-Labour defectors are now finding in their turn, new colleagues do not readily embrace a convert. With honourable exceptions, their old colleagues reserve a special venom for them. Like the gang of four who formed the SDP in 1981, and assorted politicians who defected or gave up during Labour's long rehabilitation, Prentice got little credit from those who stayed behind to fight - or from those leftwingers who had fought him, stayed behind and have slowly metamorphosed into Blairites.
Prentice would have chuckled at some of his old foes in sharp suits and sombre ties. In later years, he was president of the Conservative Association in Devizes, where Tory chairman, Michael Ancram is the MP. He lived in Marlborough and listed walking and golf among his recreations.
He is survived by his wife, Joan, and a daughter, Christine.