The US government is considering whether this year’s presidential and congressional elections could legally be delayed in the event of a terrorist attack that threatened to disrupt the ballot.

The Department of Homeland Security has asked the Justice Department’s office of legal counsel to report back on the legal requirements for postponing the November elections, in the event of a terrorist attack, according to a homeland security official.

The request came last week as senior US officials stepped up their warnings that al-Qaeda was planning attacks. Tom Ridge, homeland security secretary, said: “Credible reporting now indicates that al-Qaeda is moving forward with its plans to carry out a large- scale attack in the US in an effort to disrupt our democratic process.”

President George W. Bush, speaking on Monday in Tennessee, said: “The terrorists are ruthless and resourceful and we know they are preparing to attack us again.”

The request is part of a broad review of measures aimed at increasing security for the elections. US officials have repeatedly emphasised the threat since the March 11 train bombings in Madrid, which helped defeat the Spanish government of José Mara Aznar, who had been a staunch US ally on Iraq.

The official said the US had no specific intelligence suggesting that al-Qaeda intended to attack on election day. Further, he said, the legal issues involved in delaying an election were “very complex”.

“Technically, you’re talking about amending the constitution of the US, which only Congress can do,” he said. In addition, because state and local elections occur on the same day as the presidential and congressional elections, a postponement would also require changing a raft of state and local laws.

The question of how to respond to a terrorist attack aimed at disrupting the election was raised earlier this year by DeForest Soaries, the chairman of the Election Assistance Commission, created by Congress in 2002 to help sort out the voting problems that plagued the 2000 presidential election. He has urged the Department of Homeland Security to step up efforts to work with local officials on election day security planning, and to adopt contingency plans in the event the election is disrupted.