Some of Labour’s union backers have urged Keir Starmer to wait before telling Labour MPs to support a Brexit deal, amid growing unease within the party over the leader’s policy. Several sources said Starmer was already set on backing a Brexit deal should it be agreed and return to the Commons for a vote. Some in his team believe the issue is too big for the party to abstain on, and fear opposing the deal would suggest they preferred a no-deal outcome. However, in a conference call with Labour’s affiliated union bosses on Friday, Starmer was urged by some to back away from a decision over the potential vote until the details of any agreement are published and properely analysed. With unease mounting among some MPs and peers, those opposed to supporting a deal are keen to reopen the option of abstaining in any vote. One union insider said any decision to vote for the Brexit deal would hand a campaigning slogan to the SNP in Scotland and Plaid Cymru in Wales at next year’s local elections. The pro-Remain Transport Salaried Staffs’ Association (TSSA) led the calls for Starmer to wait before deciding to endorse a deal, but other unions said Labour risked no-deal should it back away from endorsing an agreement. While Unite’s Len McCluskey is understood not to have intervened in the conference call, his union – Labour’s biggest financial backer – is said to be supportive of backing a Brexit deal to show former pro-Leave voters that the party accepts the referendum outcome. Asked about Friday’s call, TSSA’s general secretary Manuel Cortes said: “Until we see the text of a deal – if one is eventually reached – it’s too early to say which way our Labour party should whip its MPs. If we don’t know what’s in it, how can we decide whether to vote for it or not?” Rachel Reeves, the shadow cabinet office minister, has been talking to Labour MPs, peers and unions for several weeks about the prospect of a Brexit deal vote as part of a concerted effort to keep them on board, but concern is building. There is still opposition to voting in favour of a deal within the shadow cabinet, with even shadow chancellor Anneliese Dodds said to be torn. Others still unsure are understood to include shadow justice secretary David Lammy and shadow international trade secretary Emily Thornberry. Several MPs were “really pissed off” by what they saw as a concerted attempt by leadership to marginalise those with doubts about backing a deal, one insider said. Concerns have also emerged among Labour Lords. Former leader Neil Kinnock told fellow peers in a private message: “Johnson’s ‘deal’ will not cover the huge and prolonged NON tariff impediments and costs of Brexit or safeguard the industries, services, communities and people who will be harmed by that reality. If Labour supports the ‘deal’, all our future justified criticisms will be rebuffed by ‘but you voted for this!’ and, because it will be true, it will be lethal. “We must abstain and explain that this is the rational course when faced with a damaging ‘yes’ and a disastrous ‘no’. Johnson will get his ‘deal’ through the Commons anyway and he must OWN the outcome – if we share ownership, we share complicity in the harm to our country (the most vital issue) and we give him a totally unwarranted credibility.” One Labour figure who backed a second referendum said it was a miscalculation by Starmer’s team to think that “the relatively small proportion of former Labour voters who are highly motivated by Brexit will in any way be persuaded to back the party again because it is backing a deal”. They added it was also a mistake to assume the huge number of Remain voters who backed Labour in 2017 have “nowhere else to go”.