The news echoes the controversy surrounding the proposed Crossrail link through central London which, it was feared, could have placed some of the capital's most important buildings at risk of subsiding ( Crossrail could 'sink' Barbican ).
Attracting more than 5 million visitors a year, the Sagrada Família is the Catalan city's biggest attraction, but giant tunnelling machines will bore 12m shafts just metres away from the cathedral's foundations, risking subsidence or flooding.
A particular point of concern is a protective wall, which would be built just 75cm from the cathedral's foundations - although some believe the vibrations from the passing trains could also be damaging.
Speaking to The Times, Jordi Bonet, the chief architect who has spent 22 years trying to finish Gaudí's work, said the new high-speed train link 'could prove fatal' for the landmark.
'We are extremely concerned about the tunnel passing so close,' he said, adding that it could cause 'irreversible damage'.
The Spanish government and Barcelona's local authorities have tried to allay fears, claiming that the work will pose no risk to the cathedral, but several thousand locals are reported to have begun a campaign in protest against the plans.
Gaudí began work on the Sagrada Família in 1882, but his masterpiece was left unfinished after he was run over and killed by a tram in 1926. Work continued after his death and is hoped to be complete in 30 years time.