Keir Starmer is expected to announce several new appointments to his top team when he resets his leadership after the Batley and Spen byelection next month, with several longtime allies including Jenny Chapman moving aside.

Lady Chapman, who was an early backer of Starmer’s leadership bid, will be shifted from her role as his political director to be a frontbench spokesperson on Brexit in the House of Lords, Labour sources confirmed on Tuesday.

The change is expected to be part of what one Starmer ally called a “substantial” shift in his backroom operation, set to be announced next month, as he tries to overcome criticism that he has failed to put across a clear message to voters.

Starmer’s allies believe Boris Johnson could call a general election in 2023, or even as soon as next year, and are keen to throw off the constraints of the pandemic and shift into campaigning mode.

MPs have reported frustration at what they see as a bunker mentality in the leader of the opposition’s office, with unclear reporting lines and a few longstanding allies of Starmer making decisions. One MP described it as “sofa government”, while another said: “No one in the PLP [parliamentary Labour party] knows what goes on in there.”

Chapman’s job change, first mooted in the immediate aftermath of last month’s Hartlepool byelection, comes after criticism that she was heavy-handed in dealing with Labour MPs, and made a tactical misstep by advocating ardent remainer Paul Williams as the party’s candidate.

“I have never encountered someone so difficult to deal with and I went through the Corbyn years,” one senior official said. “Relations seriously deteriorated over the Liverpool mayoral selection and the Hartlepool selection.”

Despite a barrage of private grumbling about Chapman from MPs, Starmer and his closest aides had previously seemed determined to defend her because of the key role she played in his leadership campaign. Chapman once said she would occupy his office until he stood for leader.

The shake-up is expected to coincide with the arrival at Starmer’s side of Labour’s new director of strategy, the veteran pollster Deborah Mattinson, who worked closely with Gordon Brown before founding the political consultancy Britain Thinks.

The embarrassment of losing the red wall seat of Hartlepool to the Conservatives last month, with a 16% swing against Labour, was followed by a botched reshuffle that saw a breakdown in relations with Starmer’s deputy, Angela Rayner.

Rayner was replaced as Labour’s campaigns coordinator by the MP for Birmingham Ladywood, Shabana Mahmood. In a bid to sharpen up Labour’s messaging, she and her deputy, Conor McGinn, have since instituted a daily 8am call with Labour officials to run through the political priorities for the day ahead.

As the after-shocks continue to reverberate through the party, Starmer’s longstanding director of communications, Ben Nunn, is also leaving, while the chief of staff, Morgan McSweeney, will be shifting to focus on rebuilding the party machinery for the next general election. Starmer’s parliamentary private secretary, Carolyn Harris, also stepped aside last month.

One name being mooted among Labour officials for the chief of staff role is McSweeney’s deputy, Helene Reardon-Bond, a former senior civil servant who was initially brought in by Jeremy Corbyn.

“A lot of people have said to Keir, you need a person who’s at the top of the tree, and enforces a culture and a plan. Someone whose job is making everyone else work towards a shared agenda,” said one senior Labour source, who added: “Helen is a very experienced manager, without an axe to grind.”

Another name tipped by some party aides is Spencer Livermore, the Labour peer who previously worked under Brown – perhaps for the post of political director. Meanwhile, one party official said the outgoing editor of political programmes at the BBC, Rob Burley, had been suggested as a potential replacement for Nunn.

Rayner emerged from last month’s reshuffle with an enhanced role, shadowing Michael Gove at the Cabinet Office, as well as a new brief looking at the future of work.

Chapman’s new role would technically sit under Rayner – a move that has caused some amusement among MPs – as Lord Frost is a Cabinet Office minister, although he is known to report directly to the prime minister.