Sir Keir Starmer has guaranteed himself a place on the final ballot to become Labour’s next leader after becoming the first candidate to be backed by a second major trade union.

Usdaw, the retail union and Labour’s fourth largest affiliate, on Monday nominated the shadow Brexit secretary for leader and Angela Rayner for deputy leader.

Starmer had already won support from Unison, the UK’s largest trade union, and the environmental affiliate Sera, reaching the threshold to make it on to the final ballot to replace Jeremy Corbyn.

The decision will be a blow for the Birmingham MP Jess Phillips. It was widely believed that Usdaw, one of the “big five” unions, was the most likely to give her its support. She and Emily Thornberry still have no nominations from constituency Labour parties (CLPs) or affiliated groups, leaving them with a difficult path to the next round.

Responding to Usdaw’s decision to endorse him for the Labour leadership, Starmer said the party “will stand shoulder to shoulder with the trade union movement as we take on the Tories and rebuild trust with working people”.

“I’m honoured to have received Usdaw’s endorsement for the Labour leadership,” he went on. He has also won support of 11 of the 14 CLPs to nominate so far.

Starmer is the first of the five leadership candidates to win the requisite support to be on the final ballot. Party rules say a candidate has to have won the nomination of at least two trade unions plus one affiliate comprising 5% of affiliate members.

The only other route on to the ballot is by receiving nominations from at least 5% of CLPs.

Rebecca Long-Bailey is seen as favourite to win the support of Unite and the Communication Workers Union and has already received the backing of the grassroots organisation Momentum.

Sources from GMB believe that the Wigan MP Lisa Nandy may win the support of the union on Tuesday. Emily Thornberry appears to be struggling to secure union nominations.

Monday’s decision at Usdaw’s central office in Manchester was made by the union’s 15-strong executive council.

Paddy Lillis, Usdaw’s general secretary, said: “Usdaw believes that Keir Starmer and Angela Rayner are the right leadership team to unite and rebuild Labour after a devastating election loss. Our members desperately need Labour in power, they cannot afford another decade of Conservative governments attacking workers’ rights, incomes and public services.

“The Labour party must be led by someone who can persuade voters that they have what it takes to be a prime minister and we are a government in waiting. That is at the heart of Usdaw’s decision to make these nominations.”

GMB’s 50-strong executive will decide who they will back on Tuesday following a meeting and candidates’ hustings behind closed doors at the TUC’s headquarters. An announcement is expected at 4pm.

Nandy has been in contact with the union’s general secretary, Tim Roache, and her campaign is run by the MP Louise Haigh, who is a firm favourite with the union.

Starmer is still in the running for the GMB vote, sources said, because he is “a safe pair of hands”.

Nandy’s campaign received a boost last week when the National Union of Mineworkers backed her as a candidate.

The deadline has now passed for people to join Labour to vote in the leadership contest, which will end with the announcement of the winner on 4 April.

Phillips, who has admitted in the Guardian to an “awful” first hustings in Liverpool on Sunday, has today said the party must stop trying to appease “fearmongering opportunists” who sought to exploit immigration.

Writing in the Independent, Phillips said Labour needed to challenge the “pernicious myth” that working-class communities were hostile to immigration. She said Labour needed to be clear that it did not believe the numbers coming to settle in the UK were too high.