Government is almost the last closed shop in British life. Anyone who wants to be in the cabinet before 2010 had better ensure that they are elected as an MP this June. If you want to be prime minister by then, it’s far too late – you should have entered the House of Commons in 1992, if not before.

Aspiring ministers must accept a long apprenticeship in the Commons before moving slowly up the Whitehall ladder. “Outsiders” – those who have made their names in business or the voluntary sector – need not bother thinking about a political career at the highest level if they are aged over 40.

John Major was a positive newcomer, having the least parliamentary experience of any 20th-century premier – 11 years – before he took office. Even Tony Blair had been an MP for nearly 14 years before his victory four years ago. For the 19 people who made Number 10 their home from home in the last century, the average wait was 23 years.

How different this is from the rest of British life. England’s football team turns to Sweden for its manager, and the cricket team hires a Zimbabwean; the nation’s favourite airline (BA, in case you had forgotten or are Richard Branson) is run by an Australian, while Marks and Spencer is headed by a Belgian; the Millennium Dome was half-rescued by a Frenchman; and a former American CIA agent is now trying to reshape London Transport.

While some companies, such as BP, still rely on nurturing homegrown talent and developing high-flyers such as
John Browne, an increasing number look outside for their chief executives. Having recruited internally for decades the BBC, for example, has picked both its current director-general and his predecessor from the same independent television stable. Yet the PM selects the 20 or so people in the cabinet from a strictly limited field.

The ability to handle parliamentary debates and answer questions on your feet is still regarded as a key factor in the promotion of ministers. Little account is taken of management experience. That is unfortunate, since many ministers have been no good at running their departments. MPs have always been sniffy about outsiders who become cabinet ministers soon after entering Parliament, pointing to the failure of Frank Cousins, the union leader, in the 1960s, and of John Davis, the employers’ leader, in the 1970s.

Of course, there are similar traits in the upward paths of successful ministers and business executives: finding the right patrons (Gordon Brown in the present government), gaining allies, weakening rivals (Brown versus Peter Mandelson and Robin Cook) and, above all, luck.

But all this manoeuvring occurs within a small and closed world, so it is fanciful to talk of career planning at Westminster. True, ministers generally progress up the ladder from under-secretary to minister of state to cabinet member. And there have been attempts, notably under the Tories in the 1980s, to use the whips’ office as a training ground for future stars – of which Major himself was the most distinguished graduate. Yet chance often plays as big a part as careful planning.

Few ministers under Blair would claim that their career development has been planned as part of an exercise to spot and develop talent. Rather, they regard promotions and reshuffles as largely arbitrary, depending on who is in – or out of – favour at the Blair court at the time. Part of the problem is that Blair himself has never served as a junior minister. Therefore, ministers complain, he does not understand their ambitions, hopes and frustrations.

Ministerial careers also depend on the swings of electoral fortune, of course. Plenty of promising Labour MPs who in 1979 were either on the edge of the cabinet, such as Gerald Kaufman, or just in, such as Roy Hattersley, never returned to office because their party was out of power for so long. In other cases, early promise was never fulfilled.

But the most distinctive feature is the small pool of MPs from which ministers are picked. It is this prior limitation that so narrows the range of potential ministers. We have few female ministers because we have few women MPs, and this problem is even more acute for the tiny cadre of MPs from ethnic minorities.

Despite this, the Blair cabinet has had a record number of female members, and the pressure to keep this up will be strong. But the key issue has less to do with numbers than the actual jobs that these women do. Women have held social affairs or party management posts, but we have not yet seen a female chancellor, foreign secretary, defence secretary or home secretary, although I would bet that, by the end of a second Blair term, at least one of these posts will be held by a woman – possibly Yvette Cooper, who will also be a pioneer in taking maternity leave while serving as a minister.

The closed shop has one big hole: the House of Lords. It is possible for outsiders to become ministers quickly if they are made peers. In the past – with the exception of the leader of the Lords, the Lord Chancellor and perhaps a couple of others – most of them have not been policy-makers with real clout. But Blair has tried to broaden the range of expertise in his administration by giving several peers proper responsibilities.

Two former chairmen of major companies – David Simon of BP and David Sainsbury of the family supermarket chain – have served as trade and industry ministers, while Gus Macdonald, who ran a successful media group in Scotland, has got a grip on transport after three unsuccessful predecessors in only two years. And there are other examples of key jobs performed by ministers from the Lords, such as Baroness Symons, who is in charge of defence procurement, and Baroness Hayman, who has played a key role in tackling the foot-and-mouth crisis.

Usually, though, ministers in the Lords are confined to secondary roles. One reason for this is that, in general, heads of departments have to be selected from the Commons. Members of the lower house also dislike the idea of giving power to unelected ministers. It was this sentiment that finally undermined Lord Young in the late 1980s. But the real constraint is that MPs want to be able to question ministers directly in the Commons. That is how the closed shop perpetuates itself.

It is always fun to predict who will be future prime ministers. Much money has been lost this way, and very little won. The short-term favourite – a position occupied in the mid 1980s by John Moore – often fades fast. Most people think that Gordon Brown will be the next Labour leader, but don’t rule out Jack Straw and David Blunkett, who would also stand. For the long term, keep an eye on Charles Clarke, if his bluntness does not make too many enemies, or Yvette Cooper, if she has the staying power. Of course, by 2010, Labour may be in opposition.

On the Conservative side, the field is much wider. Plenty of senior Tories fancy their chances if Hague falters on 7 June. You can take your pick from Michael Portillo, Francis Maude, Ann Widdecombe, Iain Duncan-Smith and David Davis. That breadth of choice, or rivalry, may just save Hague. Then, looking ahead a decade, Damian Green could be well placed if the Tories were ever to move back to the centre.

Some predictions do come true. I remember it well when a senior Tory whip told me in 1988 how John Major was certain to be the next Tory leader. If only I had put money on it then.