The biggest threat to new Prime Minister Boris Johnson’s premiership is Theresa May , say Westminster insiders.

The former PM is poised to take her place on the backbenches when Parliament returns – and many believe she will “do a Thatcher” by running things from behind the scenes.

Insiders say that with a slim majority, Mr Johnson will struggle to get legislation through.

And with Brexit looming, Mrs May’s ­influence on the backbenches will be crucial.

Mr Johnson killed off her deal this week and said the UK was prepared to leave without one.

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And, in a move that infuriated her inner circle, he pressed for the backstop to be scrapped. Sources say she is making plans to set traps for the new leader.

Her first step was consolidating her power base.

Hours after leaving Downing Street, she watched England’s cricketers with two of her since-sacked ministers, David Gauke and Greg Clark, and ex-chief of staff Gavin Barwell.

Earlier, Mrs May was spotted wearing a gold pebble bracelet that bore a striking resemblance to one that belonged to Mrs Thatcher.

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This prompted speculation she had been the secret bidder at an auction in May. One Whitehall source said: “May might have bought ­Thatcher’s bracelet but she also plans to copy what she did when she stood down.

“She’s going to bring together all her former Cabinet ministers. She’ll plot and give them orders.

“They threaten a guerilla war against Boris. She’s angry and will get even.”

As she left No10 Mrs May said she hoped for a Brexit “that works for the whole United Kingdom” – a remark seen as a message to her successor.

The replacement of her Cabinet with hardline Brexiteers has given Mrs May a taste for revenge, insiders say.

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Within hours of the new PM taking over, 13 full Cabinet ministers and four others who attended Cabinet were sacked or resigned. The clear-out included many of her old guard.

International trade ­secretary Liam Fox and Greg Clark, the business secretary, left their roles, as did ­education secretary Damian Hinds, and James Brokenshire, the housing and communities secretary.

Sources say Mr Johnson’s strategy was a gamble – and he is relying on outgoing ministers staying loyal.

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One insider said: “They’ve fired people who they think will vote for the Government’s deal on Brexit. That’s why you’ve seen strange people brought in – they had to be bought off.

“There are loads of people Mrs May gave positions to. They owe her.”

If, as expected, the Tories lose the Brecon and Radnorshire by-election on Thursday to the Liberal ­Democrats, the Government’s working majority will fall from two to one.

It will be the smallest of any PM since John Major. The insider added: “Can you imagine getting anything through?”

Mrs Thatcher fell out with Mr Major over his handling of the economy just weeks after she resigned as PM.

They had a row in January 1991 over interest rates and she was furious about his plan to abandon the poll tax.

Brutal reshuffle strategist is real power at No10

Leave campaign architect Dominic Cummings is believed to have pulled the strings on Boris ­Johnson’s brutal cabinet cull.

The reshuffle, dubbed the Night of the Blond Knives, was the bloodiest many in Westminster remember as the new PM dished senior posts out to hardline Brexiteers.

From the outside, it looked like a shambles. But those in the know say it was a highly efficient operation and unelected official Mr Cummings is now the real power in Downing Street.

Insiders say the reshuffle bore all the hallmarks of one of his favourite tactics – known as the “OODA Loop” and used by fighter pilots.

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Special advisers have already been told they will have to answer to him.

One Whitehall insider said: “People are pissed off he’s back. He’s beyond Marmite.

“He’s a totally different substance. You either love him or you totally f***ing hate him.”

James Graham, who has interviewed Mr Cummings many times, told BBC Radio 5 Live’s Emma Barnett: “One of his main strategies is ‘OODA Loop’, which stands for Observe, Orient, Decide and Act.

“It’s this military strategy used to surprise your opponent. It’s what fighter pilots use.”

Mr Cummings, 47, has a reputation as being one of the most brilliant but divisive thinkers in Westminster.

He graduated from Oxford in Ancient and Modern History in 1994 then moved to Russia, where he helped set up an airline – despite threats from the KGB.

But the venture flopped as the airline only had one passenger and the pilot took off without him.

He then came back and worked on the campaign to keep us out of the euro. His first Whitehall job was in 2002, as strategy director for Iain Duncan Smith.

But he quit after eight months, saying his boss was “incompetent”.

He then ran a think-tank before spending two years in a bunker on his family’s Durham farm, reading science and history.

It was Michael Gove , then Education Secretary, who brought him back as chief of staff in 2010. But Mr Cummings’ time away from Whitehall had not mellowed him.

He called former Brexit Minster David Davis as “thick as mince, lazy as a toad and vain as Narcissus”.

And he branded David Cameron “a sphinx without a riddle”.

Mr Cameron branded Cummings a “career psychopath”.

Independent MP Sarah Wollaston said Mr Johnson had made a “serious error of judgement” in bringing in Mr Cummings. But our insider said: “Dom doesn’t care about stuff like that.

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His real mission is just to disrupt everything.”

Meanwhile, a clutch of Brexiteers including Zac Goldsmith, Simon Clarke and Nadine Dorries have been made junior ministers.