Scotland’s First Minister said there was a “golden opportunity” for moderates to press for continued single market and customs union membership.
It came as Ian Blackford, her party’s Westminster leader, revealed he was working with the Liberal Democrats, Plaid Cymru, and Green Party to amend the EU Withdrawal Bill at report stage.
If their bid is successful, ministers would be blocked from using so-called Henry powers to amend primary legislation and drag the UK out of the single market and customs union.
Sturgeon said: “More than 18 months on from the Brexit vote, it beggars belief that the UK Government is not only still unable to say what kind of relationship it wants with the EU, but has also failed to produce any meaningful economic assessment of the different possibilities.
“Bluntly, the hard Brexiteers have had their chance and failed.”
Theresa May has vowed that Brexit will include taking the UK out of the single market, though the Prime Minister has left herself with maximum room for manoeuvre on regulation surrounding the customs union.
Sturgeon added: “[Hard Brexiteers] have completely failed to explain how their approach could even remotely come close to replacing the enormous lost trade and investment of leaving the single market.
“That means there is now a golden opportunity for those moderate voices who are making the case for Scotland and the UK to remain in the single market.
Labour leader Jeremy Corbyn was empty-chaired after declining an invitation from Blackford to attend cross-party talks on blocking hard Brexit in Parliament last week.
Shadow Brexit Secretary Keir Starmer has said the party wants to remain “in a single market and a customs union” but does not support full membership beyond a transition period. Around 87% of Labour’s rank and file back single market membership, however, a recent Mile End Institute survey found.
Some pro-EU Labour MPs, such as Chuka Umunna and Chris Leslie, could choose to back the cross-party amendment. However, the Parliamentary arithmetic would require all Labour MPs and Remainer Tories to rebel against their leaders.
A Labour Party spokeswoman said: “This amendment would not keep the UK in the single market and customs union, nor does it intend to deal with the serious flaws in this Bill.
“Labour has been clear that we want a time limited transition on the same basic terms we have now. Following that, we want a final deal that retains the benefits of the Customs Union and the Single Market. Options for achieving that are subject to the negotiations.”
Lib Dem Leader, Sir Vince Cable, said: “This amendment is a significant step in the fight against Brexit, demonstrating the increased willingness of parties to work together formally rather than just behind the scenes. Through the four leaders in the House of Commons, we are presenting a united front. We urge the leadership of a fifth opposition party, Labour, join us so we can change course from the dangerous Brexit being pursued by the Conservatives.”
Plaid Cymru Westminster Leader, Liz Saville-Roberts, said: “Through this important amendment, the four opposition parties are presenting the UK Parliament with an opportunity to change course, away from this needlessly damaging Brexit. We remain hopeful that the Labour Party will work with us and take the opportunity to defeat the Tories.”
Green Party Co-Leader, Caroline Lucas, said: “It’s heartening to see parties working together to halt the calamitous Tory Brexit plans. What happens this year in Parliament will have huge and lasting repercussions for the country – which is why we’re putting party differences aside and working alongside each other. We urge the Labour leadership to join us in backing this crucial amendment – and show that they too are willing to put the national interest first.”
Scottish Government analysis, published on Monday, sets out the impact on Scotland’s future economic growth of three options for the future UK relationship with Europe – if the country cannot remain members of the EU.
The study – Scotland’s Place in Europe: People, Jobs and Investment – looks at continued membership of the single market, a preferential trade agreement or non-preferential access on World Trade Organisation provisions.
A UK Government spokeswoman said: “We are seeking a deal that works for the whole of the UK, that delivers on the result of the EU referendum.
“Rather than trying to undermine the result of a democratic referendum, we urge the Scottish Government to work with us to ensure, as we leave the EU, we protect the UK’s vital internal market.
“Scotland trades four times as much with the rest of the UK as it does with the EU, so it is vital that we ensure that market continues unimpeded.”