The rival campaigns for the Labour leadership have become embroiled in a war of words over fears centrist MPs could launch a “semi-split” which would see them attempt to claim the mantle of official Opposition in Parliament.

John McDonnell and Kate Green, the respective chairs of the Corbyn and Smith teams, traded criticisms this weekend over who was stoking disunity in the party.

McDonnell, the shadow Chancellor, claimed his former shadow Cabinet colleague Owen Smith had failed to denounce those MPs who were calling for a breakaway and accused him of trying to win votes through “scaremongering”.

“Owen Smith refused when asked… to condemn the minority of MPs supporting his campaign who are talking of splitting our party if he does not win. And then only later that day in a TV interview he talked up the threat of a split. If Owen truly wants to unite our party like Jeremy Corbyn does, then he needs to denounce those who are plotting to divide it…

“Owen Smith therefore needs to immediately distance himself from those people saying they want a split, which is causing huge damage to our party at this time. Anything short of this will make him the ‘disunity candidate’.”

McDonnell’s intervention prompted fury from the Smith team, however, with former frontbencher saying McDonnell was using “hollow words on party unity” as well as pointing to the 2012 interview when he described the Labour Party as simply a “useful vehicle”.

“The irony of John McDonnell offering hollow words on party unity will not be lost on Labour members and supporters.

“Since the devastating referendum result, Owen has worked tirelessly to unite the party, negotiating with Jeremy, with trade unions, and with people across the Labour movement in an effort to bring all parts of the party together. While John, who previously referred to the Labour Party as just a ‘tactic’ , has remained relaxed about the prospect of the party splitting.”

The row broke out after long-standing rumours finally surfaced in a report that centrist MPs are considering a “semi-split” that would see them approach the Speaker of the House of Commons in an attempt to be named the official Opposition.

It is possible – although far from certain – the MPs could seek to claim the Labour Party name and assets but stop short of a formal split.