With 98.45 per cent of the unofficial count completed, Felipe Calderón, the centre-right candidate for the ruling National Action Party (PAN) still leads his leftwing rival.

But as the official count began on Wednesday morning – a winner is expected to be declared within a maximum of four days from Wednesday – it emerged that the addition of about 2.6m votes omitted from the provisional count had narrowed an already small lead of 402,000 votes to a razor-thin 257,000 or 0.6 per cent of the total.

“It is a photo finish,” a high-ranking official at Mexico’s Federal Electoral Institute (IFE) told the FT on Wednesday.

On Tuesday, a top PRD congressman questioned the missing votes, suggesting they were just one of several “inconsistencies” to have arisen in the count. But on Wednesday, IFE explained that they were excluded in line with rules agreed by all Mexico’s political parties prior to the election.

The continuing uncertainty about who will become Mexico’s next president is likely to worry Mexico’s business classes, who had initially celebrated Mr Calderon’s slim lead at the beginning of the week. The stock market went up almost 5 per cent following the news on Monday, and the peso saw its biggest one-day rise in six years.

Ever since campaigning began in January, investors – particularly local investors – have expressed deep concern about Mr Lopez Obrador’s plans to ramp up social spending and his insistence on renegotiating the North American Free Trade Agreement (Nafta) with the US and Canada.

Many also worry about what they regard as the candidate’s scant regard for legal process, and point to Mr Lopez Obrador’s record during his term as Mexico City mayor as proof.

By contrast, investors see Mr Calderon, a 43-year-old Harvard-trained economist, as a guarantee of existing, market-friendly policies that have stabilised the economy and paved the way for rising levels of foreign investment.

In Wednesday’s official count, IFE officials and party representatives began reviewing the paperwork recording the results from the country’s 130,000 voting stations. In the event of any dispute packets containing the physical paper votes will be opened and the votes recounted.

Officials insisted that this would give candidates the opportunity to resolve several discrepancies that have emerged between private estimates and IFE’s own provisional numbers.

On Tuesday, the FT reported that Mr Lopez Obrador’s Democratic Revolution Party (PRD) was “almost certain” to mount a legal challenge. Manuel Camacho, a key figure in Mr Lopez Obrador’s campaign, said he was considering legal action over several other aspects of Mexico’s long and bitter election campaign.

These centered on allegations that included possible violation by the PAN of maximum permitted limits on campaign financing and President Fox’s partisan comments throughout the campaign.