Warnings that Labour might face a 1980s-style schism have been cropping up since Jeremy Corbyn’s leadership election took off.

Yet we ought perhaps to take this morning’s more seriously, for its author is none other than Lord Owen, one of the founding members of the Social Democratic Party (SDP) which splintered from Labour in 1981.

He has told The Times (£) that if Corbyn wins, any precipitous move to unseat him could trigger a breakaway as left wingers realise there is no space for them in the Labour tent.

This threat is more plausible than it might at first appear, because the groundwork for such a split has already been done.

Despite Ed Miliband implementing some sensible reforms to his party’s relationship with the unions after the Falkirk selection scandal, their leaders still enjoy a capacity for organised mischief unlike anything else in British politics.

A year ago the Unite boss was talking (hypothetically, of course) about establishing a breakaway ‘Workers’ Party’, and it has previously been noted that Len McCluskey and other hard-left trades union barons have laid the foundations for such a split by sponsoring a caucus of MPs

Today’s Sun (£) reports that no fewer than 39 of the 48 Labour MPs who voted against welfare reform have received cash donations from the unions.

Moreover, the unions’ hands have been strengthened by Miliband’s tenure as leader: his legacy has been to shift the balance of opinion leftwards both amongst the Parliamentary Labour Party (PLP) and the membership.

It may be that the unexpected Tory majority makes a formal split less likely: McCluskey had envisioned allying with the Liberal Democrats to pass electoral reform and make the Workers’ Party more viable. That scarcely seems likely now.

Yet the existence of this nascent party within a party could still be immensely problematic for the new Labour leader (assuming it isn’t Corbyn) in the years ahead.

Barring a real shock, the left wing candidate is going to wildly exceed expectations when the votes are counted in September, whilst Liz Kendall – the only candidate seeking an unequivocally reforming mandate – is set to come fourth.

If this happens, the left is inevitably going to feel vindicated and energised. It isn’t difficult to imagine the red barons bestowing Corbyn, now a high-profile darling of the grassroots, with the unofficial captaincy of their pet caucus to ‘hold the leadership to account’.

The result is either a Labour leadership beset by ongoing insurrection, or a Labour leadership that keeps its head under the parapet and makes little progress toward bringing Labour back to electability – assuming it doesn’t go backwards.

Some in Labour are mooting the rather forlorn tactic of pulling a bait-and-switch in 2019, installing a charismatic reformer too close to the election for the shine to come off. But it’s difficult to see which candidate could possibly compensate for three or more years of the sort of press the Corbynite guerillas could inflict on the party.

Worse, if the Labour left keep collaborating with the SNP – as the full party already have, to block a fox hunting amendment – it will only make Conservative warnings of a Lab-Nat pact that much more potent in 2020.