The French president, Emmanuel Macron, has stressed his opposition to a “blind Brexit” in which the UK leaves the EU without clarity on the terms of a future trade deal, fearing that pushing the issue down the line could lead to an extension of the 21-month transition period.

A number of EU member states, including Germany, believe an aspirational political declaration on the future that ducks the most contentious trade issues now would be the best way to avoid a no-deal Brexit.

The priority for the remaining 27 EU member states is to have the withdrawal agreement – on citizens’ rights, the Irish border and the £39bn “divorce bill” – completed and ratified by 29 March 2019.

EU and British negotiators will then have until 31 December 2020 to sign off on a future trade deal, during which time the UK will effectively remain a member of the EU, but without voting rights.

However, in recent months, Macron’s diplomats in Brussels have insisted clarity in the political declaration – a non-binding document, accompanying the withdrawal agreement and spelling out the basic principles of the trade deal to come – is vital to avoid problems further down the line.

In Paris, it is feared that maintaining the unity of the 27 members on trade after the withdrawal agreement has been signed could be significantly more difficult.

Without clarity now, internal EU negotiations could eat into the 21-month transition period after Brexit day, even before talks start in earnest with the UK, said an EU diplomat privy to behind-the-scenes discussions.

“It is not a matter of France being isolated on this, but they are the ones pushing it,” said the diplomat. “They are always talking about the need for clarity and precision.

“Two years would not be long enough to have a wider internal discussion and negotiate with the Brits. The transition would need to be extended. France has had enough.”

The political declaration accompanying the withdrawal agreement only requires a qualified majority of member states to support it before 29 March 2019. Every member state will have a veto on the final trade deal when it is negotiated during the transition period.

A number of members, including Hungary, Poland, Belgium, the Netherlands and Portugal, are said to have made the case in the past few months that the EU could be more generous in response to the UK’s proposals to ensure the free flow of goods after Brexit.

Last week, the Polish foreign minister, Jacek Czaputowicz, told reporters that he would try to persuade his fellow EU ministers “to be more open to the arguments of the UK”.

However, these interventions have been fleeting and generally gone against the grain of the discussions.

With member states effectively wielding a veto during debates on the future trade deal during the transition period, the internal EU position could be more fraught, France fears.