Initially scheduled for the beginning of the week, the publication has been delayed until Friday (16 September), one day after the general elections in Denmark where border controls are a favourite topic of the populist Danish People's Party.

"Since the free movement of persons within the area without internal borders is a key Union achievement, the benefits of which are enjoyed by all the persons living in this area, it should as a general rule require a decision to be taken at the Union level, rather than for such decision to be taken unilaterally at the national level," the paper reads.

The proposal comes after member states earlier this year asked for clarification on when and how border checks can be reintroduced, following a Franco-Italian spat over Tunisian migrants crossing their common border.

Home affairs ministers could not agree, however, what role to give Brussels in the decision-making process, so the EU executive stepped into the breach.

In the commission's view, "unilateral national initiatives, in isolation, can never be an effective response to common threats" in the EU, where more than a billion people cross freely from one country to another every year.

Therefore, the "main rule" laid out in the proposal is that "any decision on the reintroduction of internal border controls would be taken by the commission as an implementing act involving the member states accordingly."

The European Parliament would be informed of such measures, which would be for renewable periods of up to 30 days, with a maximum length of six months.

Greece clause

Additionally, the EU commission and the bloc's border control agency Frontex, would organise snap checks of the 25 members of the Schengen area to see how they are managing the borders.

If shortcomings are found, governments will have to fix them. Failure to comply may trigger, "as last resort", the reintroduction of border checks with that particular country. There is no time limit foreseen in that case.

No countries are mentioned specifically in the proposal, but last year Greece had to appeal to the EU and fellow Schengen states to help out with guarding its land-border to Turkey, the main entry point of migrants from the Middle East and north Africa.

The kick-out clause is also meant to reassure enlargement-sceptic countries that once Bulgaria and Romania are admitted in Schengen, the process is not irreversible.

Motives for reintroducing border checks can vary from a terrorist attack to football championships or high-level summits. An "unexpected inflow" of migrants can also trigger the reintroduction of border controls, provided there is a risk assessment proving that they may be a public safety concern.

In urgent cases, member states can set up border checks on their own, but only for a maximum of five days, after which they have to come up with a detailed assessment of why the measure is necessary and ask for permission from the EU commission.

Foreseeable events for which checks are needed have to be flagged up "at least six weeks in advance with all relevant information relating to the reasons for doing so, and regarding the planned scope and duration."

Since the Schengen border code came into force in 2006, border checks have been introduced 26 times and never for longer than 30 days. In most cases, it was because of major sporting events, political rallies or high-level summits.

"However, the information member states are required to provide to other member states and the commission often does not arrive in sufficient time, or contain sufficient details, to enable the commission to usefully issue an opinion concerning the notification," the draft reads.

But since Germany, France and Spain on Tuesday clearly indicated they do not agree with ceding sovereignty to the EU in this area, the proposal is likely to be watered down in negotiations with member states and the European Parliament.