Thatcher judged best prime minister (by a left-wing historian)

Margaret Thatcher was the best prime minister of the 20th century, according to a left-wing author and historian who argues that Tony Blair's aspirations to the title have been wrecked by the Iraq war.

In an article in the BBC History Magazine published today, Francis Beckett puts Baroness Thatcher at the top of his list because she "took one sort of society, and turned it into another sort of society". "She broke the [post-war] Attlee settlement, which had lasted more than 30 years, largely by force of will," he wrote.

"Today few people under 40 remember a time when trade unions were a real force in the land; when the public sector controlled large swaths of the economy; when local councils controlled education and other local services; when benefits were considered rights of citizenship. The defeat and destruction of the once-powerful National Union of Mineworkers was a key moment of the last half-decade."

Clement Attlee, whose 1945-51 Labour government introduced the modern welfare state, shares the top spot with Lady Thatcher with a maximum five points. Perhaps surprisingly, they are ahead of Winston Churchill, Britain's war-time leader, who scores only four points - the same as Sir Edward Heath, the Tory prime minister who took Britain into the European Union in 1973 but lost three general elections.

The prime ministers of the last century were judged by Mr Beckett on their effectiveness as "change managers". He judged whether they had a clear idea of how they wanted to change Britain, how far he succeeded in doing so and how effective they were at simply managing, rather than creating, change.

Mr Blair is placed in mid-table with three points despite winning three elections. Mr Beckett argues: "Blair made a lot of progress in his chosen direction right up to the time of the Iraq war. The private sector has now been brought even into the running of schools and hospitals, and since the Conservatives agree with it, this will probably be a relatively permanent change.

"The unpopularity of the Iraq war, and the fact that the reasons given afterwards for going to war were not those given at the time, have undermined Blair's ability to implement his vision, probably permanently." Neville Chamberlain, the Tory prime minister from 1937-40, is rated the worst of the century, alongside Anthony Eden, who resigned after the Suez crisis 50 years ago.

Mr Beckett wrote: "He [Chamberlain] utterly failed in his principal objective of averting war. The moment which appeared at the time to be his biggest triumph - the Munich agreement with Hitler of 1938 - is seen in retrospect as a disaster. He said at the time that it was 'the prelude to a larger settlement in which all Europe may find peace'. He never had the chance to do much about his domestic agenda."

John Major, who served seven years before losing to Labour in 1997, scores only one point.

Dave Musgrove, the editor of BBC History Magazine, says: "The important point here is that we're not judging these leaders on their policies, but rather on how well they implemented them ... Mrs Thatcher undoubtedly scores highly on that front. I know many people take issue with what happened to the nation when the 'Iron Lady' was in power, but no one can deny that she did what she set out to."

Best ... and worst

* [Rated 0(worst) - 5 (best)]

5 Margaret Thatcher 1979-90

5 Clement Attlee 1945-51

4 Edward Heath 1970-4

4 Winston Churchill 1940-5, 1951-5

4 Harold Macmillan 1957-63

4 Sir Henry Campbell-Bannerman 1905-8

3 Lord Salisbury 1885-92, 1895-1902

3 Herbert Asquith 1908-16

3 David Lloyd George 1916-22

3 Stanley Baldwin 1923, 1924-9, 1935-7

3 Harold Wilson 1964-70, 1974-6

3 Tony Blair 1997-

2 James Callaghan 1976-9

2 Arthur Balfour 1902-5

1 Andrew Bonar Law 1922-3

1 Ramsay MacDonald

1924, 1929-35

1 Sir Alec Douglas-Home 1963-4

1 John Major 1990-7

0 Anthony Eden 1955-7

0 Neville Chamberlain 1937-40