Emily Thornberry, shadow Foreign Secretary, and Sir Keir Starmer, shadow Brexit Secretary, have written to David Davis in advance of an Opposition Day debate on Wednesday demanding that Parliament should be given a vote on the government’s Brexit plan before Article 50 is triggered. Labour has also requested answers to 170 key questions.

Dear David,

At the Conservative Party conference last week, the Prime Minister announced that the government would trigger the Article 50 process before the end of March 2017.

After today, that gives a maximum of 170 days before Britain must begin negotiating both the terms of our exit from the European Union, and also our future relationship with our European partners on a host of issues, ranging from trade arrangements to law enforcement cooperation.

The Labour Party has consistently argued that, before embarking on those negotiations:

the government needs to set out a clear plan of what it is seeking to achieve, and how it intends to accomplish that plan;

that plan needs to be the subject of proper consultation with the devolved administrations, overseas territories, the Mayor of London’s office, and the Official Opposition; and

the public’s elected representatives in Parliament should have the opportunity to vote on whether that plan is the right one to safeguard the country’s best interests post-Brexit.

None of the above is intended to stop the Brexit process from taking place, or to disregard the instructions we have been given by the British people. On the contrary, it is simply to ensure that we get the best possible deal for Britain, and one which reflects all the views of the communities that we as Members of Parliament represent.

It is also to ensure that, contrary to all public statements we have heard to date, the government actually has a clear plan of what it is intending to achieve, and that all members of the Cabinet with responsibility in this area subscribe to that same plan.

To that end, we have set out below a large number of unanswered questions about the government’s plan: 170 questions in total, across a wide variety of areas, to match the number of days after today before you must begin to present that plan to our European partners, according to your own self-imposed deadline.

If you are able to provide satisfactory answers to all these questions, just one per day from tomorrow until 31 March next year, it might give some confidence that the government is entering the Article 50 negotiations with a clear plan.

If not, it will reinforce the sense that the government is instead blundering into this process without a clear endgame in mind, repeating exactly the same mistake that the previous Prime Minister made with his ‘renegotiation’ of Britain’s EU membership last year: working to an artificial, self-imposed timetable; with a flawed Plan A of what he wanted to achieve; and no Plan B whatsoever.

As you yourself said in July: “The negotiating strategy has to be properly designed and there is some serious consultation to be done first. This is one of the reasons for taking a little time before triggering Article 50.”

And yet, tomorrow we will be just 170 days away from the PM’s deadline for triggering Article 50, and there is still no sign of this negotiation strategy, or the plan it is intended to deliver. Nor is it good enough to refuse to disclose the government’s plan, as you have done previously, on the grounds that it must be kept secret.

As the Director-General of the CBI, Carolyn Fairbairn, said last week: “The government’s desire to play its negotiating cards close to its chest must be tempered by clear indications on how we will trade with the UK’s most important partner and how firms will be able to employ the people needed to drive growth.”

Speaking this week, the Conservative chair of the Treasury Select Committee, Andrew Tyrie, said: “British interests will be best served by an early and full and detailed explanation from the government of what its negotiating position is before it embarks on those discussions. What has never been discussed in any depth is what we arrive at.”

I would also urge you to reflect on the words of the Japanese government in its open letter to the UK government, published before the recent G20 summit in China:

“Uncertainty is a major concern for an economy; it evokes a sense of anxiety, causing volatility in markets, and results in the contraction of trade, investment and credit. What Japanese businesses in Europe most wish to avoid is the situation in which that they are unable to discern clearly the way the BREXIT negotiations are going, only grasping the whole picture at the last minute.

“It is imperative for the UK and the EU to regain the confidence of the world and ensure their unwavering competitiveness by increasing the predictability of the BREXIT process, ensuring the outcome is free of unpleasant surprises and reducing the risks emanating from uncertainty. From this perspective, we strongly hope that the UK and the EU will present to the world the whole picture of the BREXIT process as early as possible.”

It was quite right that the official UK response to that letter was to say: “It’s not unhelpful to…have some of our trading partners setting out some of the issues they are looking at and the types of concerns or issues that they would want to be addressed. This is all more information that helps to inform our thinking on what is the right deal for Britain”.

However, to do that thinking in private while refusing to share the UK’s plan in public ignores the basic essence of Japan’s appeal, and creates further and deeper uncertainty as Japanese companies investing in Britain, and other major players in this process both at home and abroad, begin to wonder whether there is any plan at all.

We hope you will resolve that uncertainty as a matter of urgency by providing answers to all the questions set out below, and the many others that have been raised with your department, and to do so in good time before 31 March next year.

Finally, given you have consistently spoken up throughout your career in a highly principled way about the importance of Parliamentary sovereignty, we hope you will reflect again on the decision to deny the country’s elected representatives the opportunity to debate and vote on the government’s plan for Brexit before Article 50 is triggered.

Again, this would not be for the purpose of blocking the Brexit process, but simply to ensure that process will lead to the best possible outcome for Britain, and that the government’s proposed plan will deliver that outcome.

As the former Attorney General, Dominic Grieve, has said: “The idea that a government could take a decision of such massive importance without parliamentary approval seems to me to be extremely far-fetched.”

We hope you will listen to Dominic, and stay true to your principles, rather than following the edicts of an increasingly authoritarian Prime Minister, who seems intent on repeating her predecessor’s mistakes, whatever the calamitous results.

Yours sincerely,

Emily Thornberry Shadow Secretary of State for Foreign Affairs

Keir Starmer Shadow Secretary of State for Exiting the European Union.