A US government-funded institute specialising in the analysis of security threats has found that rail tunnels connecting Manhattan to mainland NewJersey could be flooded within hours if they were attacked by even relatively small bombs.

The analysis, based on work by the Lawrence Livermore National Laboratory and leaked to the New York Times, suggested that the network of tunnels was more vulnerable than had been thought. A bomb that could be carried easily on to a train could make a 50 sq ft (about 4.6 sq metre) hole in the side of the tunnels and potentially breach both sides, the analysis found. More than 1m gallons of water would enter the tunnel every minute, putting at risk the lives of up to 900 passengers - the capacity of a crowded train. About 230,000 people travel every day through four train tunnels that lie along the bed of the Hudson river. The concerns over the fragility of these tunnels are thought to apply equally to several rail tunnels that connect Manhattan to Brooklyn and Queens under the East River.

According to the analysis, the tunnels are largely made of cast iron without further protective lining. Only about a quarter of the length is reinforced with concrete or brick. The New York Times said the report was leaked by an official who was anxious that not enough was being done to respond to its findings.

The threat of an attack on the underwater tunnels has been high on the agenda of security agencies for many years. In July a plot to flood the system with the use of backpack bombs was allegedly uncovered and arrests were made in Lebanon. The FBI said the plot was the "real thing" and that it had been intercepted by monitoring internet chat rooms. The plan, according to investigators, was to place up to 20 suicide bombers in a single carriage of a train for maximum effect.