Germany’s largest centre-left party says it would be prepared to let the country’s own short-term economic interests take a back seat and make the “unity of the European Union” a priority in the upcoming Brexit negotiations if it were to form part of the next government.

According to a draft resolution seen by the Guardian, Germany’s Social Democratic party (SPD) also insists that British membership of the single market should be tied to both accepting free movement of people and recognising the rulings of the European court of justice, stating that “without recognising this [the ECJ’s] final say a membership of the single market, especially in services, will not be possible”.

The SPD, junior partners to Angela Merkel’s Christian Democratic Union (CDU) in Germany’s coalition government, is also understood to be considering a push for a parliamentary vote on the terms of Britain’s exit from the EU, a demand that could considerably delay the timetable following the expected triggering of article 50 by the British prime minister, Theresa May, in March.

The draft resolution states that “disadvantages such as loss of foreign investment in UK or disruption of supply chains” should be seen as “one-off write-offs”, arguing the collapse of the EU and end of the single market “would be significantly larger in scale” and amount to “the end of the European peace order”.

According to the text, authored by the party’s deputy parliamentary leader, Axel Schäfer, “short-term interests of individual groups or member states, including Germany” should therefore “take a back seat” during the negotiations.

The SDP is currently second in polls to the CDU, making it likely to play a part in the formation of Germany’s next coalition government after federal elections in September, either in another “grand coalition” with the centre-right or a so-called “R2G” coalition with the left and green parties.

The resolution, which is likely to be passed at the end of this week, would not only strengthen Merkel’s uncompromising stance regarding the indivisibility of free movement and the single market, but also highlights the growing rift between the British Labour party and the continental European left.

Jeremy Corbyn’s Labour party is planning to host an “alternative Brexit summit” in London at the end of February in London, with view to “build alliances with colleagues around Europe about the kind of economic and political relationship we’ll have with Europe after we leave the European Union”, as the Labour leader stated on the BBC this week.

But unlike Labour under Corbyn, the German centre-left has stated it is very much still wedded to freedom of movement for EU citizens as a point of principle. “Anyone who wants to be part of the single market has to recognise these principles,” Schäfer told Reuters on Wednesday.

While stating that Germany should strive for “friendly and close relations” between Britain and the EU27, the SPD’s draft also warns that “making this possible requires realistic expectations”, and “there are currently doubts whether this is the case on both sides”.

“If we allow a ‘Europe à la carte’ it would lead to incalculable domino effects which would threaten the cohesion of the union,” the draft said. “Refusing such wishes has nothing to do with punishment, but answers to the widely understood interests of the EU-27 and the functioning and political balance of the single market.”

The Social Democrats’ positioning coincides with a survey indicating that German business remains relaxed about negative consequences of Britain leaving the union. According to a study by the Cologne Institute for Economic Research, more than nine out of 10 German firms do not expect strong effects from Brexit on their business activities in the near future.

The survey of 2,900 firms found that only a small share of 2-3% of companies believed Britain’s departure from the bloc of states would have strong negative consequences on their investment and employment. Meanwhile, a quarter of German firms expected to benefit from diversion of business activities away from the UK.

The Social Democrats are also believed to be weighing up a push for the German government to get a vote not just on a new trade deal between the EU and Great Britain, but also the conditions of Britain’s exit deal – a move that could set an example for other national governments around Europe to emulate.

Current plans only foresee the exit deal to be passed by the European council and the European parliament, with national parliaments in the 27 EU member states to get a vote on a deal outlining the new relationship between Britain and the EU, or a transitional deal once the process of the UK leaving the bloc of states has been completed.

Putting the agreement outlining Britain’s exit from the EU to the vote in Germany would therefore prove controversial, not least after the difficulty European member states recently experienced in building a positive consensus around the EU-Canada Comprehensive Economic and Trade Agreement.