France and Germany have proposed that countries in Europe's Schengen border agreement be given more leeway in suspending the pact in extreme cases, a proposal likely to fuel debate in Europe.

In a joint letter to Denmark, which holds the rotating presidency of the EU, the French and German interior ministers said amendments to Schengen, which permits visa-free travel across 26 EU and non-EU countries, were necessary to ensure tighter security within the European Union.

Issued days before the French election, in which President Nicolas Sarkozy has campaigned on the need for tighter border controls to limit illegal migration, the letter will be seen by critics as having a clear political purpose.

Member states should have at their disposal, as a last resort, the possibility of reintroducing internal border controls for a period not exceeding 30 days, says the letter, written in French and obtained by Reuters.

At the end of those 30 days, it will be up to the European Commission to recommend to the (European) Council to continue or interrupt the internal border controls.

EU states already have the power to allow the reintroduction of border controls for 30 days if there is a specific threat to security or public order, and have done so on multiple occasions, such as during the 2006 soccer World Cup in Germany.

The European Commission has proposed limiting the suspension by national authorities to five days, with any extension contingent on approval by member governments after a proposal by the Commission.

But the Franco-German letter says the power should remain in the hands of member states for up to 30 days, without the Commission having oversight. It also aims to ensure that countries can reintroduce borders to stop a flow of migrants into the bloc, one EU diplomat said, in cases when a country is failing to sufficiently guard external borders.

The idea, the diplomat said, is to add the possibility for a member state to reintroduce borders because of a persistent deficiency or a massive influx.

The idea is likely to find favour in some EU states, but opponents see it as thinly veiled criticism of countries in south and eastern Europe, which have been accused of not policing borders adequately, allowing in illegal migrants.

Immigration is a heated topic in France's presidential election. Sarkozy has threatened to suspend France's participation in Schengen if the rules are not strengthened, a bid to shore up his support among right-of-centre voters.

The Franco-German letter may be discussed among EU interior ministers at a meeting in Luxembourg next week, although for now it is not formally on the agenda. Denmark, as holder of the EU presidency, and the European Commission have said the issue will not be discussed until June.

Schengen involves all 27 EU countries apart from Britain, Ireland and Cyprus.

Romania and Bulgaria have complied with the treaty and are expected to join it soon. Outside the EU, it includes Iceland, Norway, Switzerland and Liechtenstein.

(Writing by Luke Baker; additional reporting by Ilona Wissenbach and Justyna Pawlak)