Jeremy Corbyn has personally intervened to give greater power to thousands of new Labour members who backed his leadership.

In a move that displays a fresh willingness to empower the party’s vast army of newer members, the Labour leader and his allies backed a rule change allowing many more of them to help to choose candidates in one of the key battles in next year’s local elections.

Corbyn supporters said that he “put his foot down in the name of party democracy” by demanding that anyone who had been a member since about the start of the year would be able to select the party’s council candidates in Birmingham.

Previously, members would have had to have joined before summer 2015 to have a vote. It would have meant that few of those who voted for Corbyn in his two leadership bids would have had a say, making it harder for leftwing candidates to be selected.

According to several sources, Corbyn surprised colleagues by attending an obscure internal committee meeting last week to back the effort to hand a vote to newer Birmingham members. Some warned him that, in effect, he would be overruling decisions already taken by the local party. However, the reform was secured after it was put to a vote.

It represents a significant move because Birmingham will be voting for an entire 101-strong council next year. The West Midlands has also been regarded as a Labour area with a large number of critics of the Corbyn leadership. The shift in the rules means the party’s left is set to gain significant power in the region.

The move has been interpreted by some as evidence of the growing confidence of Corbyn and the left to press ahead with reforms that strengthen the power of new, pro-Corbyn members. A party spokesman said: “We don’t comment on internal selection processes.”

It comes after internal battles over control of the party burst open again last week, with MPs again fearing that they will face deselection by emboldened Corbyn supporters. His team have been trying to calm fears after Luciana Berger, the MP for Liverpool Wavertree, was told to “get on board” with the leader by one new party official.

Yvette Cooper, the former shadow home secretary, warned the party’s left in a speech on Saturday not to engage in abuse of internal opponents or the Conservatives. “The attacks, abuse and attempts at intimidation don’t just come from the right,” she said. “Sometimes our members and supporters target each other. Frankly, Labour party members should be united in supporting Luciana Berger, not targeting her or trying to intimidate her – always something that should be unacceptable within Labour; even more shameful to target someone who has stood up to fascists, and especially when she is on maternity leave.

“​Nor should we accept​ vitriolic abuse against opponents. Shamefully, during this general election campaign we’ve seen Tory women MPs and candidates targeted with vile abuse from the left. And Labour supporters at rallies holding placards with the severed head of Theresa May. Maybe it was meant as a joke. It isn’t funny.”

There are now longer term plans by the left to push ahead with attempts to give more power to members. The aim is to push for a series of radical changes at Labour’s conference in 2018.

Extensive changes being canvassed include handing Labour members power to nominate leadership candidates – meaning the left would be guaranteed to have a candidate in any race. Currently, any candidate needs the support of 15% of Labour MPs or MEPs.

Another would make it easier for local members and trade unions to instigate a selection process ahead of an election should they have any concerns about the incumbent MP.

The left’s next aim is to gain control of the committee that controls which proposals will be debated at the 2018 conference. The conference arrangements committee elections take place later this year and the left is well placed.

The committee has five union representatives and two other elected positions, held by MP Gloria De Piero and peer Michael Cashman, both of whom have been critical of Corbyn’s leadership.