A series of bomb scares brought deadlock to the center of Britain's highway network Thursday, halting exports from the north to Channel ports for most of the day.

Prime Minister John Major said during a campaign visit to Scotland that the action appeared to be the Irish Republican Army "playing silly games," adding: "Yet again they have shown the nature of the people they are."Business leaders said the attacks appeared to be aimed at causing maximum disruption to the regional economy. The costs to exporters, suppliers and truckers was estimated to run into "millions of pounds."

The disruption began with a series of early morning telephone calls threatening terrorist attacks on highways near Birmingham and Northampton in the English Midlands. The bomb scares severed the main road links between northern and southern England.

Police in the West Midlands confirmed they discovered "a viable bomb which could have caused considerable loss of life" on an elevated section of the M6 highway. It was detonated by an army bomb disposal team in a controlled explosion.

Police in Northamptonshire also discovered a suspect package under the M1 highway near Daventry.

"It is not just the frustration caused by these bomb scares, it is the cost," said Tony Bradley, director of policy at the Birmingham Chamber of Commerce. "Distribution represents around 15 percent of the costs of an average company in our region, and the impact of these events is substantial."

The highways closed Thursday - the M6, M5 and M1 - are among the most congested in Britain.

Thursday's disruption mirrored last week's attacks, for which the IRA claimed responsibility, on railway routes in northern England.