Human error was likely to blame for the breakdown of the world's largest atom-smasher, the European Organisation for Nuclear Research (CERN) said Monday.

One of the 10,000 connections that join the 27-kilometre (16.9-mile) Large Hadron Collider (LHC) likely overheated and made a hole that leaked helium into the tunnel, the head of the project at CERN, Lyn Evans, told a press conference.

The repair team, however, had not yet been able to confirm this, he said.

The leak happened September 19, crippling the atom-smasher, which will not be up and running before the end of April 2009, CERN said.

Buried underground in Geneva, the LHC took nearly 20 years to complete and at six billion Swiss francs (3.76 billion euros, 5.46 billion dollars) is one of the costliest and most complex scientific experiments ever attempted.

It aims to resolve some of the greatest questions surrounding fundamental matter, such as how particles acquire mass and how they were forged in the "Big Bang" that created the universe some 13.7 billion years ago.