Shadow chancellor John McDonnell has said he does not trust the Prime Minister, as he accused her of jeopardising the cross-party Brexit talks for her own "personal protection".
Following newspaper reports that Theresa May is preparing to give ground this week in the discussions, the Labour frontbencher said the PM had "blown the confidentiality" of the talks.
He said his party wanted to get a deal done "as soon as possible" but needed guarantees that an agreement would not be "ripped up" by a future Conservative leader.
And asked on BBC One's Andrew Marr Show if he trusted Mrs May, he replied: "No, sorry, not after this weekend when she's blown the confidentiality I had and I actually think she's jeopardised the negotiations for her own personal protection."
Mr McDonnell accused Mrs May of acting in "bad faith" after the Sunday Times reported that the Prime Minister would put forward plans for a comprehensive but temporary customs arrangement with the EU that would last until the next general election.
And he said: "We are negotiating with Theresa May's team as requested. Whilst we're doing that and we think we're gaining an understanding of our different positions and where we can reach some compromise, in the wings, if you like, are all the leadership candidates virtually threatening to tear up whatever deal that we do.
"So we're dealing with a very unstable Government and let me just use this analogy: it's trying to enter into a contract with a company that's going into administration and the people who are going to take over are not willing to fulfil that contract. We can't negotiate like that."
Mr McDonnell also said it "may well" be the case that any deal would have to be voted on in a second referendum, adding: "I think the Conservatives have to recognise that if a deal is going to go through there might be a large number of MPs who will want a public vote."
Earlier, shadow health secretary Jon Ashworth said it was "disappointing" that it appeared Tory ministers and spin doctors had been "briefing about what's happening" in the talks.
"We've entered these negotiations in good faith – they should be confidential at this stage because if you want to get an agreement you have to be able to respect the position of those sat around the table and we seem to be reading all kinds of things in the newspapers today," he told Sky News' Sophy Ridge On Sunday.
"I would say to those Tories negotiating this isn't really the best way to go about it to be frank."
Labour's deputy leader Tom Watson said it would be "very difficult" to agree a deal in the cross-party talks.
"Obviously for a large number of our colleagues a confirmatory ballot on a deal is a deal breaker. Now it might be we find some form of deal that diminishes that desire but it will be very hard to achieve and only time will tell on that," he told BBC Radio 5 Live's Pienaar's Politics.
"But I don't think we should give false hope on this, it's going to be very difficult to find a negotiated settlement."