Delays to repairs intended to ease traffic chaos in Brussels have been blamed on hungry mice, who have apparently eaten the construction plans.

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

The Belgian capital’s regional parliament has been told that repairs are being held up because the original construction plans have been destroyed – apparently eaten by rodents.

The tunnels provide vital routes across what is often described as Europe’s most congested city. But, for decades, the construction plans were stored in the pillars of 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 officials on Wednesday.

The state of the roads in the city of 1.2 million people, home to the EU and Nato headquarters, has become a hot political issue in Belgium, with an estimated bill of €1bn (£780m) to repair all the tunnels.