The June 24-25 gathering in The Hague comes as US President Donald Trump demands other NATO members shoulder a greater burden and Russia's grinding war in Ukraine forces Europe to bolster its own security capabilities.

Germany and Poland are among the countries supporting the goal of five percent of gross domestic product, up from a two-percent goal set in 2014, but Spain has resisted its allies' calls to go further.

"For Spain, committing to a 5 percent target would not only be unreasonable, but also counterproductive," Sanchez told NATO chief Mark Rutte in a letter written in English.

Sanchez said each NATO member needed to invest different sums to meet their military capability targets, with the military estimating that 2.1 percent would suffice for Spain.

Spain does not want to limit other NATO allies' spending ambitions but seeks "a more flexible formula" at the summit, Sanchez said.

This declaration could recognise each NATO ally's different path to its capability target, making the five-percent spending target optional, or exclude Spain from the new goal, Sanchez suggested.

In 2024, Spain was the NATO member that dedicated the smallest proportion of its annual economic output to defence and found itself in the firing line of Trump's ire.

Sanchez has announced more than 10 billion euros ($11.5 billion) of fresh defence investment to hit the two-percent target this year.

But he faces a balancing act of aligning with NATO allies and cajoling his junior coalition partner, the far-left alliance Sumar, which is hostile to increasing military spending.

Sanchez argued accelerating spending would rush Spain into defence purchases that could "exacerbate equipment interoperability challenges" and prevent European suppliers "from developing their own industrial base".

A spending spree risked weighing down economic growth through higher debt and inflation and diverting investment from areas such as health and education, he warned.

"If we truly want to increase real spending in a sustained way, our main goal should be to ensure that our economies grow significantly."