When David Cameron took Britain’s MEPs out of the centre-right European parliament party the European People’s Party (EPP) ten years ago, little was thought of it in the UK beyond the then narrow confines of the eurosceptic movement. The decision to leave the pro-European grouping in favour of a more eurosceptic grouping – the European Conservatives and Reformists in European Parliament – can be largely understood as an attempt to appease the anti-EU wing of the Tory party. That was a move rooted in Cameron’s desire to hold his party together ahead of the 2010 general election in the UK.

The decision had wider ramifications that have rippled down the years. Cameron effectively took Britain out of an alliance with powerful European players in countries such as France and Germany – and locked Britain out of formal channels of discussion with other European centre-right parties.

Now, with a Brexit extension and the UK’s participation in the upcoming MEP elections looking all but inevitable, the decision could lead to a whole host of different problems for the UK – and for the EU.

Thanks to the political fragmentation of the continent and the changing topography of European politics the stable centre-right grouping of the EPP does not have the same foothold it once had – and with the UK’s participation in the elections, ushering in 75 non-EPP MEPs, if the British do take their seats in the event of a very long delay, the grouping’s grip on European Parliament looks potentially even weaker.

Why this matters to the UK – isn’t the UK leaving, at some point? – is a fair question. Well, it matters because of the identity of the next President of the European Commission. The UK will be negotiating with said person for some time to come.

The weakening of the EPP could herald an entirely chaotic European Parliament after May’s elections. MEPs will meet post-election in July against a backdrop that is bound to feature some grandstanding from President Macron. Amid the chaos, there is a decent chance we’ll see the upending of the Spitzenkandidat system by which the President of the Commission is chosen.

Current president Jean Claude Juncker was elected via the Spitzenkandidat system – a process whereby every party in the European Parliament fields their lead candidate (Spitzenkandidat) for the presidency. Naturally, the candidate of the party who holds the most seats in parliament takes up the mantle of President of the Commission. In the case of Juncker – that’s the dominant EPP. And as such it makes the EPP’s new candidate – Manfred Weber – look like a near shoe-in.

However, some on the Council, who have to ratify the new president, don’t like the system and certainly don’t like Weber – someone who has never held a ministerial position in his native Germany. He is considered to be weak on rising populism. His critics say he is basically not up to the task. As the EPP’s position – and therefore the authority of its Spitzenkandidat – is already in peril and further destabilised by the UK’s weird participation in the election, the council could be given a cue to consider upending the entire system.

If the EPP doesn’t gain a comfortable majority in the elections then it really threatens the legitimacy of their candidate. Macron is specifically hostile to the system. He could simply use his council veto and run a public campaign to block Weber’s ascension to Commission President. Then there’s even the prospect that the Council will abandon the process and via direct or indirect mechanisms, just pick a president themselves with disregard for the parliament’s will.

And what kind of European Commission President would other members of the Council want to see? Someone who can command the confidence of member states, who can cross party lines, who can work with socialist governments as easily as right-wing ones, someone who is both a pragmatist and a centrist, someone who can get on with Germany’s CDU and Macron’s en Marche all the same.

Who on earth could that be? Step forward Brexit negotiator, France’s Michel Barnier. Despite Weber being considered a shoe-in, right now, a Barnier presidency is in prospect.

This would be an interesting development for the UK, facing the figure who led the EU’s side of the Brexit negotiations. He has observed the British side up close and has the British government’s number, it seems. Barnier seems unlikely to shift position in the years ahead of Brexit talks.

Today, Barnier warned that if there is no deal with the UK, then the EU will take a hard line and refuse to do any trade deal with Britain until London has paid the £39bn exit bill, guaranteed citizens rights and provided satisfaction with an Irish border backstop. In other words, no progress without going back and signing the existing Withdrawal Agreement terms.

In Germany, Weber’s party is advocating a softer line. It may count for nothing if Weber is usurped in Brussels.

This is awkward. The British are, to put it mildly, out of favour with the EU as it is. Cameron leaving the EPP began a process that weakened the European People’s Party and led to the current state of affairs.

Brexit looks set to drag on for years, and the second phase of negotiations seem as far away as the hazy memory of the 2016 referendum seems distant. It is impossible to know exactly what kind of effect the forthcoming European elections will have. But there is a rich irony here.  Cameron’s decision to appease his eurosceptics back in 2009 is still having an effect on the entire Brexit process and on the future of the European Union.