The Warsaw-based agency launched last week with plans to have a rapid response pool of 1,500 border guards at its disposal before the end of the year.

The agency, an evolution of Frontex, has been mandated to assess border weakness in an effort to avoid a repeat of last year's large migratory and refugee inflows.

Leggeri, who described the so-called vulnerability tests as a "kind of performance audit", said the methodology behind the future probes will most likely be adopted next week.

The three EU states will be used to set up a baseline in order to create a benchmark for the upcoming controls.

Should the agency detect a problem, then it can launch more targeted tests.

"It is about anticipating potential difficulties and preparing measures in order to deploy more equipment, more operations, if needed," he said.

Such tests are not limited to sudden large migration flows. They may also assess, for instance, whether an EU state is able to detect fraudulent documents.

If the EU state fails to plug the gap assessed by the agency, then the Council of the EU, representing member states, could reintroduce internal border controls for up to six months.

Leggeri told MEPs the tests complement but do not replace the so-called Schengen evaluation mechanism, which also monitors external border management in the Schengen free-travel zone, which covers 22 EU states and Iceland, Norway, and Switzerland.

Schengen mechanism

An European Commission official said that Schengen evaluation is different because it is designed to see if member states comply with the zone’s border-free rules.

Last year, the commission carried out 21 visits to EU states on Schengen compliance. Some 16 have been held so far this year, with a team sent to Austria unannounced this week.

"It comes really as a surprise and I'm very sure for Austria that it also came as a very much as a surprise," the official said.

An announced visit to France is also underway this week. Results will be kept confidential.

Portuguese centre-right MEP Carlos Coelho, who has had access to the results, said there was no reason to keep them hidden from the public.

"Ninety or 95 percent of the content doesn't justify to be classified," he told the committee on Wednesday.

Austria, Germany, Denmark, Sweden and Norway are allowed to maintain temporary border controls until mid-November due to the migration crisis.

But the commission aims to lift the controls before the end of year as part of a “back to Schengen” plan.