A year after the EU referendum and 12 weeks after article 50 was triggered, formal negotiations over Britain’s exit from the EU opened today.

Britain needs a good Brexit deal to safeguard jobs, security and trade and to build a new partnership with the EU. Achieving this will be fiendishly difficult. The sheer range of issues that need to be negotiated and resolved is daunting – from security, trade, customs and immigration to citizens’ rights, our legal system, the Irish border and fisheries. That is why it is all the more important that Britain enters negotiations with a clear mandate, settled priorities and strong alliances with our EU partners.

Sadly, that is not the position we are in. The prime minister’s mistakes and miscalculations have meant that Britain enters negotiations without a mandate at home or alliances abroad. It is the worst possible basis from which to start negotiations.

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Theresa May’s decision to call an unnecessary general election after article 50 was triggered was deeply irresponsible. It wasted vital weeks that could have been spent preparing for today’s talks – weeks that the European commission spent settling on and setting out their Brexit priorities with stark clarity.

The prime minister’s failure to secure a mandate or a majority has also caused huge uncertainty about the government’s Brexit approach. The chancellor clearly advocates a softening of strategy. Yet Brexiteers in the cabinet and on Conservative backbenches will fiercely oppose any such change of emphasis. May lacks the authority to either drive through her stated Brexit approach or to change course. In the weeks to come she will have to choose. No wonder the German foreign minister has spoken of a “difficult, even impossible situation”, where the prime minister is “without clear majorities and [a] clear negotiation strategy”.

The belligerent approach May has adopted has also served to weaken our relations with the EU, alienated our allies and left Britain short of goodwill across Europe. This really matters, because, if we are to deliver a Brexit deal that works for Britain and brings a strong new partnership with the EU, we will need to work with and not against the other 27 member states.

That is why last week I wrote to David Davis urging the government to change course and to make four very simple but vital changes.

First, to adopt a much more constructive and responsible tone and approach – one that highlights our shared history and values and points to a shared future between Britain and the EU.

Second, to rule out “no deal” as a viable option. As Labour has said for months, leaving the EU without a deal is the worst possible option: it would be disastrous for our economy, for jobs, for our security and for our ability to engage in a whole range of everyday transactions with the EU.

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Third, we need a Brexit approach that has jobs and the economy at its heart and places much greater emphasis on the benefits of the single market and the customs union. The prime minister has been far too quick to take options off the table and far too willing to put concerns over immigration above the economy. This has to change.

Finally, May must drop her evasive and closed approach and ensure that parliament is far more closely and regularly involved in the process. At every stage, the prime minister’s instinct has been to push challenge and scrutiny away, a flaw that could prove as costly in negotiations as it was in the general election. The composition of the new House of Commons, however, offers an opportunity to reach across and include parliament in the process.

Today’s negotiations are the first stage in a long and complex process. Let us hope the prime minister learns from her mistakes and makes the changes needed to deliver a Brexit deal that works for Britain. If she does not, Labour stands ready.