Tiny fibres from old green baize cloths on the counting tables caused at least 10 machines to break down after they became electrostatically charged and the dust collected in the scanners. The chief returning officer, Rob Hughes, admitted: "We've had a few machines that have gone pear-shaped."
The 168 machines, which have never before been used in a UK election, were due to produce a final tally by 5am yesterday, but the build-up of dust and static meant the result came seven hours later.
The machines, which resemble large photocopiers and are already used to mark students' multiple-choice exam papers, were introduced because a traditional manual count would take at least three days, while each machine is able to count 150 ballot papers a minute.
Despite a smooth full-scale mock election in February and numerous real ballots, including the 1998 elections in Bosnia-Herzegovina, yesterday all but two of the 12 machines in the Enfield and Haringey constituency broke down, while another stopped briefly in Bromley, bringing the rate at which papers were counted down from 40,000 an hour to just 27,000 an hour.
Mr Hughes said: "Dust in the air is always something difficult to solve. It caused static and stopped the machines working. They have just got bunged up. This is a brand new system of voting and a brand new system of counting. It is not surprising we have had a few problems."
Engineers from the machine supplier, Data & Research Services, were called in and, 90 minutes later, identified the problem as the old, dusty green baize covers on the counting tables. A spokesman said: "Tiny fibres got electro-statically charged and got caught up in the papers, collecting on the scanners. We stripped down the machines and cleaned the scanners and everything was fine again.
"We've never had this problem before. We've operated these machines in a typhoon in Bangladesh before without any problems."
The machines - which use sensors to detect where crosses have been placed - also failed at times to separate and count correctly the two parts of the ballot papers, while one machine was reported to have overheated and another was damaged when a table collapsed. Problems were exacerbated by incorrectly completed papers, creating a long backlog needing individual adjudications by election officials and candidates' agents, and also folded papers, which the machines cannot read.
