Analysis of Boris Johnson’s election campaign trail has found he has been defending Tory marginals – such as Iain Duncan Smith’s Chingford and Woodford Green – rather than the Labour seats he needs to win to secure a majority in the House of Commons.

The prime minister’s visit to the former Tory leader’s Essex constituency – where recent polls show Duncan Smith being pressed hard by Labour – has bolstered suggestions of nerves in Conservative high command over potential losses in Remain-backing Tory seats in London and the southeast.

Other high-profile Brexiteers in true-blue seats thought to be facing an unaccustomed threat include foreign secretary Dominic Raab in Esher and Walton and veteran eurosceptic Sir John Redwood in Wokingham. They are both being targeted by Liberal Democrats, who believe Mr Johnson’s “get Brexit done” message is going down badly with Conservative Remain voters.

Mr Johnson has made a major play of his decision in the final three days ahead of the 12 December election of taking the fight to seats in the Midlands and the North where concentrations of Leave support have made traditional Labour strongholds vulnerable for the first time in decades.

But despite national polls giving Tories a comfortable lead averaging 10 points, analysis by Tortoise magazine shows that the prime minister has spent most of his time in Conservative-held constituencies, in what appears to be an effort to shore up existing MPs rather than seize new territory.

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Constituency polls have indicated that some big Tory names such as Mr Duncan Smith could be under threat.

Chingford and Woodford Green voted to Remain by a wafer-thin 50.12 per cent in the referendum, and Labour is mounting a fierce campaign behind candidate Faiza Shaheen to unseat the Brexit figurehead in a seat where he won a majority of less than 2,500 in 2017.

Constituency-level analysis of polls involving almost 290,000 voters, released over the weekend by Datapraxis, named the former Tory leader’s Essex constituency as one of 17 battleground seats where Labour are challenging incumbent Conservatives, with Ms Shaheen at 42.4 per cent against Mr Duncan Smith’s 46.6 per cent.

Liberal Democrats claim to have detected “seismic” shifts in true-blue seats such as Wokingham, as well as South Cambridgeshire, previously held by former Tory Heidi Allen, where they believe Conservative Remainers are deserting Mr Johnson’s party in large numbers over his “get Brexit done” message.

Most high profile of these seats is Mr Raab’s Esher and Walton, which Jo Swinson has suggested might deliver a “Portillo moment” by falling out of Tory hands for the first time in decades. Lib Dem challenger Monica Harding has been buoyed by the endorsement of Mr Raab’s predecessor as Tory MP Ian Taylor in a seat that voted 58 per cent Remain in 2016.

Lib Dems are also hopeful of taking Remain-backing seats in the capital such as Richmond Park, where Zac Goldsmith is defending a majority of just 45, as well as Wimbledon and Kensington.

Ahead of his foray into the Labour heartlands of Grimsby, and Washington and Sunderland West on Monday morning, Mr Johnson and his battlebus had remained firmly in Conservative territory over the past week.

Friday saw him in Finchley and Golders Green, a constituency that Lib Dem Luciana Berger is hoping to prise from Tory hands for the first time since 2010.

And other Tory seats he has travelled to include Lib Dem target Cheadle and Conservative-Labour marginals Bolton West and Milton Keynes South, as well as what would normally be seen as rock-solid “holds” such as Salisbury, Derbyshire Dales, and Rochester and Strood.

Chris Cook of Tortoise mapped the prime minister’s battlebus itinerary onto a chart showing Tory-held seats on the left and Labour on the right. Leave-voting seats are higher up the chart and Remain seats are lower, according to the strength of their 2016 result.

He found that, until Sunday, Mr Johnson’s visits were concentrated in the upper left quarter of the chart – containing Tory seats that voted Leave, where the party would be expected to be most confident of winning.

By contrast, there were no Labour-held seats on the tour over the last week, he said. Over the same period, Jeremy Corbyn’s visits – marked with a red cross – took in several Labour seats but also pushed much deeper into Conservative areas.

(Chris Cook/Tortoise) ((Chris Cook/Tortoise))

“It is striking that … Johnson is spending a lot of time making defensive visits,” said Mr Cook. “It’s possible we’ve missed some stops but we cannot find him visiting a Labour-held seat this week. Last week, he went to a Labour-held seat in Plymouth – but not to campaign. He went to a statue-unveiling.

“I’d highlight that the last time we have him on a straight campaigning visit to a Labour-held target was when he paid visits to Sedgefield and Stockton in County Durham on 20 November.”

He added: “Here’s another curiosity. Corbyn has been going to the same sort of narrow Tory holds. If you closed your eyes to the polling, the two parties are behaving as though the Tories are losing seats to Labour.