Delays to repairs to ease traffic chaos in Europe's congested capital Brussels have been blamed on hungry mice.

EU leaders gathering for a summit on Thursday to deal with the refugee crisis and British threats to quit the bloc find a city struggling to cope with repeated closures of key road tunnels caused by crumbling concrete and years of decay.

Now the Belgian capital's regional parliament has been told that repairs are being held up because original construction plans have been destroyed -- apparently eaten by rodents. The tunnels provide vital arteries across what is often described as Europe's most traffic-congested city.

But for decades the plans for their construction were stored in the pillars under a motorway bridge, for want of space elsewhere. "They may have been eaten by mice," the former head of the city's infrastructure agency told city lawmakers on Wednesday.

The state of the roads in the city of 1.2 million, home to the European Union and NATO headquarters, has become a hot political issue in Belgium, with an estimated bill of some 1 billion euros to repair all the tunnels.