The day June 15 has a noble history to it. It was on that day in 1215 that King John affixed his seal to the Magna Carta. A rather more obscure historical event may, though, set a precedent that is more relevant to British politics. It was on June 15, AD923, that Charles the Simple, King of France, son of Louis the Stammerer, nephew of Charles the Fat was brutally deposed from power at the Battle of Soissons, imprisoned and died from his maltreatment some six years later.

Not many people would describe our Prime Minister as “the Simple”. Gordon the Complex would be more accurate. Yet the date - before any general election - when Mr Brown is at most danger of suffering this ancient French king's fate (minus imprisonment and maltreatment; even the Labour Party in a blind panic is not that bad) is next year on Monday, June 15, 2009.

Some insist that he will meet a sticky end much sooner. The consensus among the commentators is that the Prime Minister is washed up and has been so ever since last autumn's election that never was. Such a belief is the product of short memories.

It is only ten weeks ago that the Conservative lead in the opinion polls was a slim 3 per cent and the smart money was on Ken Livingstone to win again in London. It is not the election that never was but an economy that is up the spout that is the issue.

The real mistake last October was not ducking the hustings but the Pre-Budget Report that came after it when the Government tried to compete with a Tory pledge on inheritance tax, enraged the non-doms (which should have been aggrieved at George Osborne, the Shadow Chancellor, for turning the spotlight on them in the first place) and the business community, while spectacularly failing to defuse the 10p tax band bomb before it became explosive.

Mr Brown is thus as safe as houses this June. To move against him now would be madness. It would make the Labour Party seem ludicrous. It would do nothing to ease the international economic situation - the core of the Government's woes - which this Prime Minister is plainly better placed to have influence than any successor. It would also be deeply irrational timing as the rocky state of the economy almost obliges a 2010 general election and no new leader could expect a honeymoon to last two years.

Which is why June 15, 2009, is the pivotal date in the political calendar. In 2004 the local elections were not held in May but in June to coincide with the battle for the European Parliament. The objective was to increase turnout, particularly for the Euro elections, which had fallen to a humiliating low of 24 per cent five years earlier. It worked, and although it has not been officially announced it is assumed widely in Whitehall that the elections will be combined again to occur on Thursday, June 11, 2009.

This has the whiff of an unavoidable disaster for the Labour Party for two reasons.

The first is that this lot of local elections involves, predominantly, English counties. These seats were last fought in 2005, on the day of the general election, and before then 2001, on the day of the general election, and 1997, on the day of the general election.

In all of these contests, turnout in these counties was far higher than normal for local politics (64 per cent in 2005) and Labour benefited from the general election effect (in 2005, the proportion of the local vote was Labour 34 per cent, Tory 31 per cent and Lib Dem 27 per cent). So even if the Government is less unpopular 12 months hence than it is today, much lower turnout in far less favourable conditions means that Labour's sizeable contingent of county councillors are on the electoral equivalent of death row. They will be slaughtered. Their number could easily fall from about 575 to half that tally. The scale of the carnage will become obvious by the middle of the afternoon of June 12.

The second reason involves the European Parliament. Although the ballot boxes will close on June 11, the EU mandates that counting cannot start until the following Sunday night because most EU states vote on Sundays. In 2004 the Conservatives underperformed badly in Europe because UKIP won more than 16 per cent of the vote, drawing support mainly from the right of the spectrum.

Since then, however, UKIP has fallen foul to factionalism and there will not be a massive EU controversy such as the debate about British membership of the euro or the proposed former constitution to boost it's cause. It is a sure bet that the UKIP share of the vote will fall dramatically and the Conservative edge over Labour will increase sharply. There will be other effects, too - such as that of Nick Griffin, of the BNP, becoming an MEP (courtesy of the proportional representation system). All of this will be clear on June 15, 2009.

If there is ever to be an occasion when the Labour Party loses its head and turns upon the Prime Minister, then this will be the trigger for it. It is the sole scenario that I can foresee - bar prime ministerial ill-health - in which Labour enters the next general election under the command of a David Miliband, an Alan Johnson, a James Purnell, anybody except Mr Brown.

The Prime Minister can, nevertheless, clutch at another unusual historical example as consolation.

June 15 next year will be the 150th anniversary of the so-called Pig War, when Britain and America almost came to blows after the shooting of a large black boar found eating in a garden of an island, that both the the US and the British thought was theirs. It seemed as if it might cause a war but common sense prevailed and the animal was the one casualty of the conflict. Mr Brown has to hope that his June 15 passes off as peacefully.

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