It’s a meeting that’s so top secret that those attending – at a secret location – are ordered to surrender their mobile phone and any other electronic devices when they arrive.

They’re given numbered copies of the agenda, thick bundles which are then collected from them at the end of the meeting. Security is extremely tight. Nothing is left to chance.

No, this is not a meeting of spooks and generals to plot the nation’s secret strategy for going to war. At least not a real war. And those participating are not members of the government. Yet.

This is Labour’s “Clause V” meeting, attended by Sir Keir Starmer and his shadow cabinet, senior backbench MPs, top trade union leaders and members of the party’s national executive.

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And it’s happening this Friday, with just one huge item on the agenda: agreeing the manifesto that Sir Keir is expected to present to the country on 13 June, three weeks before polling day.

The manifesto will be based on Sir Keir’s five “missions” launched last year – on the economy, the NHS, crime, climate change and education. Labour is declaring war – on the Tories.

Friday’s meeting is a far cry from the smoke-filled rooms and beer and sandwiches of the Labour and trade union folklore of yesteryear. These days it’s strictly mineral water and even vaping is banned.

About 80 people in all will be attending, making the task of keeping the contents of the manifesto under wraps a nightmare. And security will be tighter than ever before after the 2017 manifesto leaked.

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Labour’s shadow cabinet members, totalling 31, are pretty well known these days and include established figures like Angela Rayner, Rachel Reeves, Yvette Cooper and Ed Miliband.

Then there are the rising stars like Wes Streeting, Bridget Phillipson, Peter Kyle and Shabana Mahmood and Blair/Brown-era veterans like Pat McFadden, Hilary Benn and John Healey.

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Other attendees on Friday will include the parliamentary Labour party chairman John Cryer, the general secretaries of the 11 unions affiliated to the party and the 40 or so members of the national executive, the NEC.

The NEC is dominated by the unions, which have 11 places. Local parties have nine, plus two councillors, and veteran ex-ministers Sir George Howarth, Dame Margaret Beckett and Dame Angela Eagle are also members.

All of which makes the Clause V meeting – called Clause V because it’s the section of the party rulebook that outlines the policy making process – pretty unwieldy, though it’s likely the manifesto has already been stitched up by Starmer allies before the meeting.

But it’s from the leaders of the big unions that Sir Keir is likely to face the toughest opposition on Friday. Unions are demanding no backtracking on workers’ rights and are fighting a proposed ban on North Sea oil and gas drilling.

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Sir Keir’s union critics include the no-nonsense Sharon Graham of Unite and left-wingers Matt Wrack of the Fire Brigades Union, Mick Whelan of the train drivers’ union Aslef and Dave Ward of the Communication Workers Union.

Writing in The Guardian this week, Ms Graham declared: “I make no apology for holding Labour’s feet to the fire on workers’ rights – no matter how uncomfortable it might make some or what criticism is laid at my door.

“Of course I want a Labour government, but that doesn’t mean I will sit on the sidelines and applaud while it caves in to the business lobby and rows back on its commitments.

“The Labour leadership’s penchant for reneging on promises has been a theme of its time in opposition.”

And Mr Wrack, currently TUC president, said on Wednesday looking ahead to Friday’s meeting: “We will want to hold an incoming Labour government to account on what’s in that manifesto.”

And warning Sir Keir on workers’ rights, he said: “I don’t expect him to backtrack, it’s a vote winner and it will substantially improve the lives of millions of people.”

The Clause V meeting is expected to start at midday on Friday. It’s a pivotal moment in Labour’s election campaign. High noon, in fact.