The parliamentary Labour party has been hit by a rare outbreak of unity – over plans for a new centrist political party.

The new movement has been secretly under development for more than a year and is said to have secured £50m from financial backers, including the multimillionaire founder of LoveFilm.

“The project appears to be the most significant attempt to challenge the main parties yet to emerge,” the Observer reported.

But Labour MPs were quick to dismiss the plans, with the shadow chancellor leading the charge by focusing on the money.

John McDonnell tweeted: “That’s a novel idea. A party of the rich, by the rich, for the rich. A party for the few not the many.”

Similarly, shadow Cabinet Office minister Jon Trickett stated: “A new political party with £50m in the kitty, no members, no rule book, no ideology. Perhaps with support from sections of the British Establishment. A plaything for the rich? Let’s focus on the task in hand: building a social movement which will change our country for good.”

One-time Labour rising star Clive Lewis argued: “What you see here is an establishment reboot job. With the Tories on self-destruct mode and Labour at last offering a radical alternative to neoliberalism - this group wants a re boot and rebrand for a failed ideology.”

And backbencher Danielle Rowley also joined in, noting: “This new party are planning to ‘break the mould’ and shake things up... by receiving £50m from 'a network of entrepreneurs, philanthropists and donors'.”

Other Labour MPs took a different line of attack.

Shadow health secretary Jon Ashworth insisted that only Labour could save the NHS: “For goodness sake, what a daft waste of time. Anyone putting money into this nonsense would be better off investing in our campaign to restore and rebuild our NHS.”

Bootle MP Peter Dowd claimed the newcomers were copying his own party's policies anyway: “Reports suggest that the new party’s ‘potential policy proposals include asking the rich to pay a fairer share of tax, better funding for the NHS and improved social mobility.” Are they already Labour’s policies??!’”

And – inevitably – there was the odd mention of the SDP.

Sefton Central MP Bill Esterton hit back: “This is SDP mark 2. Mark 1 gave Thatcher three election wins. This can only keep the Tories in power for a generation.”

And the arch-Blairite and SDP founding member Lord Adonis said: “Labour and the Labour movement is the only means to defeat Brexit and bring about a radical reforming government. Believe me, I was a founder member of the SDP: it failed and we can’t fail this time, the stakes for the country are too high.”