Labour has rejected proposals for a Norway-style relationship with the European Union because the party is too divided on the issue, the shadow Brexit secretary has said.
Sir Keir Starmer has ruled out backing measures that would see the UK remain in the European Economic Area (EEA) after Brexit and instead proposed an alternative plan.
The official Labour amendments to the Brexit Bill call on Theresa May to make maintaining "full access" to the EU "internal market" an objective of the negotiations with Brussels.
Party critics have accused the frontbench of "bailing out" the Prime Minister from a potential rebellion on the single market.
But Sir Keir said the EEA amendment to the EU (Withdrawal) Bill that will go before MPs on Monday was too divisive in the Parliamentary Labour Party (PLP).
He told BBC Radio 4's Today programme: "The difficulty with that, and I think everybody recognises this, is that there are very strong and very different views across the PLP on that particular amendment.
"So, whilst there's unity on all the others and we will all be voting together, on that amendment there are very divided views."
Asked if he was putting party unity before his beliefs about what is right for the country, he replied: "I'm injecting some honesty about where we are in the Labour Party."
Sir Keir said Labour MP Chuka Umunna and other party critics of the approach knew "that this amendment does not command that support in their own party".
"I wish I could report complete unity on all amendments but we are not in that position."
Sir Keir said the amendment was "very contentious" and claimed it was a "pretence" that everyone in the party was in the same place on it.
"It really doesn't help," he added.
Former shadow chancellor Chris Leslie said: "We should not be in the business of bailing out Theresa May when she's facing rebellion on the single market."
Bermondsey and Old Southwark MP Neil Coyle added: "Many of us will be supporting the EEA amendment from Lords. Not any lesser alternative."
Ex-cabinet minister Ben Bradshaw said Tory MPs would not vote for a Labour frontbench amendment and if the party is "serious" about avoiding a hard Brexit "we must vote for the existing backbench cross-party Lords amendment".
Chuka Umunna - a supporter of the pro-EU Open Britain campaign - said: "All the way through the passage of this Bill, the only amendments which have commanded support on both sides of the House and passed are cross-party backbench ones.
"So, if we are serious about 'protecting full access to the internal market of the EU' and ensuring 'no new impediments to trade', logic dictates Labour MPs should be whipped to support the cross-party EEA amendment sent to us by the House of Lords."
Brexit minister Suella Braverman said: "Labour have shattered their promise to respect the referendum result - this amendment means accepting free movement and continuing to follow EU rules with absolutely no say in them, which is the worst of all worlds."
Brexit Secretary David Davis is set to make a speech on his hopes for the future security partnership between the UK and the EU.
The intervention comes after a bruising row over the Galileo satellite programme and European arrest warrant which saw Mr Davis suggest Brussels was concerned with "public posturing and scoring points" rather than resolving the issues.