On Tuesday, the prime minister will give a speech on Brexit. It is another chance for her to spell out what her government is hoping to achieve in the negotiations.

The stakes are high and difficult judgment calls will have to be made. But time is running out. The phrases “Brexit means Brexit” and “There will be no running commentary” are well past their sell-by date. Where there is uncertainty, we now need clarity.

No one expects the prime minister to disclose details on Tuesday that could reasonably be judged to damage the UK in the article 50 negotiations. But, equally, no one buys the argument that disclosure of the government’s basic approach to the big-ticket items such as the single market, the customs union and transitional arrangements would have any harmful effect, apart, of course, from discomforting those Tory MPs who would feel betrayed on learning that their preferred direction is not shared by the government.

So what should the prime minister say?

First, she should rule out hard Brexit.

Leaving the EU without a preferential trade arrangement in place would make the UK significantly poorer. That would leave us outside and shut off from the European market of 500 million people who could buy our products and services. Reverting to World Trade Organisation rules should not be contemplated. As the CBI has said, that “would do serious and lasting damage to the UK economy and those of our trading partners”.

Such an approach would not only put our economy and jobs at risk, but would abandon our shared scientific, educational and cultural endeavours with the EU and jeopardise our collaboration on policing and security.

By ruling out this version of Brexit, the prime minister would calm nerves and reassure businesses.

But she needs to go much further than that. Having spent the past three months travelling the length and breadth of the UK talking to communities, workers, trade unionists, industry representatives and hundreds of businesses large and small, I have a clear view of their needs and concerns. And when it comes to trade, they are relatively straight forward: no drop in the ability of businesses in the UK to trade successfully with our EU partners.

They want the prime minister to fight for them as hard as she has promised to fight for Nissan. On 31 October 2016, the business secretary, Greg Clark, told the Commons that the government had reassured Nissan that it would seek trading arrangements that are “free of tariffs” and “unencumbered by impediments”. The prime minister now needs unequivocally to extend that commitment to all businesses, whether trading in goods or services. She also needs to demonstrate she understands the importance of “equivalence” in the regulated sectors. Labour is demanding nothing less. The economy and jobs must come first.

The fact that changes to the way freedom of movement rules operate in the UK will have to be part of the article 50 negotiations makes the government’s job more difficult. But we are entitled to expect the prime minister to fight hard for the best deal for our country. And the negotiations will take place during a period of considerable challenge and change for Europe itself.

Labour has consistently emphasised the importance and benefits of the single market. But it now seems highly likely that the prime minister will signal on Tuesday that she is giving up on membership. If she thinks that lowering expectations will help her in the long run, she is mistaken.

Full access to the single market is what businesses and trade unions want. If the prime minister is going down the route of a bespoke trade agreement, Labour will demand that she spells out how such an agreement would be comprehensive enough to ensure that the benefits of the single market – trading “free of tariffs” and “unencumbered by impediments” – are matched. Warm words are not enough.

The government must be open enough to provide robust impact assessments of leaving the single market or the customs union, including region-by-region and sector-by-sector analysis.

They must also be honest enough to acknowledge that any fully comprehensive trade agreement is likely to require transitional arrangements and outline what will happen in the interim to our EU contributions and to our role in negotiating new regulations.

The prime minister also needs to clear up other uncertainties. EU citizens living in the UK and UK citizens living in Europe urgently need reassurance. Their status should be sorted out before article 50 is triggered; their rights should not be a bargaining chip in the negotiations.

Equally, the prime minister needs to indicate that she will fight for a new relationship with the EU that values and maintains our joint scientific, educational and cultural work with our EU partners; that guarantees our continued co-operation in the fight against organised crime and terrorism; and that allows the UK to retain its leading position in the world, influencing and contributing to developments across Europe and beyond.

These are Labour’s demands. They are also the demands of the country. They need to be in the prime minister’s speech; they need to be in the government’s plan for Brexit.

To ensure that is the case, parliament should have a vote on the article 50 agreement that is reached at the end of the negotiations. That will provide both grip and accountability of the process and outcome.

If the prime minister is prepared to indicate her willingness to share these objectives on Tuesday, that would be a big step forward in the national interest.

Keir Starmer is shadow secretary of state for exiting the EU and MP for Holborn and St Pancras