Keir Starmer has drafted into government two leftwing critics of his stance on benefits and green investment, as the prime minister faces pressure to drop Labour’s cautious approach to reviving the economy.

Carys Roberts, the influential director of the Institute for Public Policy Research thinktank, has joined Downing Street’s policy unit as a special adviser, while Rachel Statham, who headed the IPPR’s work on public services, has been hired to lead his policy unit’s work on childcare, the early years and education.

Roberts had been among leading figures behind Labour’s now-scrapped pledge to spend £28bn a year on green investment.

When the plan was controversially ditched earlier this year, the economist warned that such a significant scaling back on investment would “undermine efforts to grow the economy and manage the transition to net zero fairly”. Roberts will advise the prime minister on climate, energy and the environment.

Statham has also been a vocal critic of Starmer’s economic caution, and argued as recently as last month that his failure to prioritise scrapping the two-child limit on benefits would ensure Labour was making a “plan for substantial increases in child poverty” over the next five years.

In the first weeks since Labour’s election landslide Starmer has been filling key ministerial appointments and recruiting expert figures to play influential roles behind the scenes in his administration.

Alongside surprise appointments for the businessman James Timpson, as prisons minister, and leading barrister Richard Hermer, as attorney general, chancellor Rachel Reeves has created a council of economic advisers chaired by the London School of Economics academic John Van Reenen.

Since taking over as Labour leader four years ago Starmer has gradually shifted the party to the political centre ground, aiming to win back swing voters and to neutralise Tory attacks that it could not be trusted with the economy – despite the risk of alienating some in the Labour base.

His government has so far prioritised sticking closely to Labour’s “no drama” election manifesto, with the cabinet minister Pat McFadden telling the BBC on Wednesday that “people shouldn’t expect us to implement the things we didn’t say we’d do” when asked why the two-child benefit limit was being kept.

However, Starmer’s appointments give an influential role to economists who have advocated a more radical stance.

As the IPPR’s executive director since 2020, Roberts oversaw the thinktank’s Environmental Justice Commission, which recommended a £30bn increase in public investment in a low carbon economy – widely seen as the catalyst behind Labour’s £28bn pledge.

IPPR has also consistently criticised the two-child benefit limit, first introduced by the Conservatives in April 2017. Statham wrote on X last month that “to keep the two-child limit is to plan for substantial increases in child poverty over the next parliament”.

It comes as Starmer faces pressure from within his own ranks to abolish the policy after official figures showed a record 1.6 million children living in families affected by it.