If Tony Blair goes in May next year, then it will be his successor who represents Britain at the crucial EU summit next June on the future of the constitutional treaty, which, one way or the other, will determine the pattern of EU politics for the next few years.

This has so far attracted little comment in the media, but it is of potentially crucial importance for the future of Europe.

Blair has been a consistent supporter of Britain playing a constructive role in Europe and he has continued to support the constitutional treaty when some cabinet ministers were luke-warm or even hostile (despite Labour's manifesto commitment to campaign enthusiastically for it). Blair has grasped that the issue is not dead and buried, as British commentators are all too quick to proclaim, and that even if the text of the treaty does eventually die a death, then (1) it is important that Britain is not blamed for killing it off (when there are others who are all too keen to blame the Brits) and (2) the issues the treaty was intended to solve have not conveniently gone away, but remain on the table.

If Brown is there, then he will have to get to grips very quickly with issues that he has sometimes shown a disdain for. He will have to overcome suspicions from our European partners that he is a closet Eurosceptic, who rarely attends the "Ecofin" council meetings and, when he does, has a reputation of lecturing other finance ministers on how much better the British economy is doing under his stewardship (which particularly irks those who are doing better or as well, including the Finns, the Irish and the Dutch who Britain considers to be natural allies in EU discussions). He also seems, for a socialist, to be overly gushing in his admiration of US labour market deregulation and lack of social protection. And of course many see him, rightly or wrongly, as the man who thwarted Blair on bringing Britain into the euro.

The discussion on the constitutional treaty will force Brown, if it is him, to come clean on where he stands. If he takes the view (which is the easy option in terms of the short run in domestic politics) that treaty reform is dead and the enlarged EU should simply muddle through with the old treaties, then he will have both made the wrong choice in favour of a downgraded EU and he will have alienated the majority of our EU partners. He will also have kicked the British objective of further enlargement of the EU into the long grass.

If, on the other hand, he were to embrace the view that Britain is constructively committed to building a more effective, accountable EU and is still supportive of the reforms contained in the constitutional treaty, then he would both maximise British influence and give a sporting chance to the reforms that the EU genuinely needs.

Of course, there are many who would wish to see him lay his cards on the table before the summit - indeed before they would be prepared to vote for him as Labour leader. There is still a strongly pro-Europe sentiment running through the Labour party and many members, including ministers and former ministers, MPs and MEPs, trade unionists and others, for whom the issue of Europe will be a litmus test of their willingness to back him or to seek an alternative.