The leadership of Unison, one of Britain’s biggest unions, has been won by a moderate candidate, shoring up the position of Labour leader Keir Starmer.

The coming year or so will see a changing of the guard in the leadership of the UK’s three largest trade unions, which wield political power through their funding of the main opposition party and by holding multiple seats on its national executive committee (NEC).

The first result, which came on Monday, saw Christina McAnea, 62, emerge as the general secretary of Unison, which has a membership of more than 1m mostly public sector workers.

This is the first time a woman has become leader of any of the so-called Big Three — Unite, Unison and the GMB — although the umbrella Trades Union Congress has a female general secretary, Frances O’Grady.

The result was greeted with relief by the Labour leadership, with Sir Keir saying he was looking forward to working with Ms McAnea when she takes up the role on January 22.

“I know you will be a brilliant representative for Unison members,” he said. “And it's a significant moment for the union to elect its first woman general secretary.”

Ms McAnea succeeds Dave Prentis, a cautious figure who has retired after 20 years in the role.

She won 48 per cent of the votes in the ballot, seeing off three rival male candidates, including Roger McKenzie, a leftwinger backed by former Labour leader Jeremy Corbyn who received only 10 per cent.

Glasgow-born Ms McAnea, daughter of a school dinner lady, left school at 16 and worked in various jobs before starting university at 22 and then working for Glasgow council.

She began working at Unison in 1993 and in 2014 was the lead negotiator during a one-off strike by NHS staff. She briefly joined the Communist party as a teenager but her views are now more moderate.

“She’s a wonderful, warm-hearted, cheerful Glaswegian and politically very pragmatic,” said one former colleague.

The second, and potentially most significant leadership battle, involves the selection of a successor to replace Len McCluskey, head of the Unite super-union.

Mr McCluskey, whose term runs to 2022, has not yet announced his departure date but the contest to replace him is expected later this year.

Mr McCluskey wielded huge power during Mr Corbyn’s leadership of Labour, with the union giving £3m to the party in the run-up to the 2019 general election. This year, however, he has cut Unite’s funding to Labour by 10 per cent in protest at Sir Keir’s shift towards the centre.

Howard Beckett, assistant general secretary and a close ally of Mr McCluskey, is seen as the “continuity candidate”. He has been a vocal critic of Sir Keir, leading a virtual “walkout” of leftwingers from an online meeting of the NEC in November.

But the union’s leftwing caucus, called United Left, picked the more pragmatic Steve Turner, assistant general secretary responsible for manufacturing, as its internal nominee.

Despite Mr Turner’s leftwing background, the Millwall FC fan has said he does not want a “war of attrition” within Labour or a “public spat” with its leader.

A third candidate to emerge this week is Gerard Coyne, from the relative right of the party, who tried to unseat Mr McCluskey in 2017 — with the support of many MPs — only to lose narrowly.

The executive of the GMB industrial union will meet next week to set out its own timetable to replace Tim Roache, who left in disgrace last year.

The GMB contest is expected to pit Gary Smith, who has been the union’s Scotland secretary for the last five years, against Rehana Azam, a national officer for the GMB since 2004.

Neither is expected to be hostile to Sir Keir, although a more leftwing candidate could still emerge.