The £5bn Large Hadron Collider (LHC) is intended to smash protons - one of the building blocks of matter - into each other
Repairing the Large Hadron Collider (LHC) near Geneva will cost almost £14m ($21m) and "realistically" take until at least next summer to start back up.
An electrical failure shut the £3.6bn ($6.6bn) machine down in September.
The European Organization for Nuclear Research (Cern) thought it would only be out of action until November but the damage was worse than expected.
It is hoped repairs will be completed by May or early June with the machine restarted at the end of June or later.
Cern spokesman James Gillies said: "If we can do it sooner, all well and good. But I think we can do it realistically (in) early summer."
Fundamental questions
The LHC was built to smash protons together at huge speeds, recreating conditions moments after the Big Bang, and scientists hope it will shed light on fundamental questions in physics.
The fault occurred just nine days after it was turned on with Cern blaming the shutdown on the failure of a single, badly soldered electrical connection in one of its super-cooled magnet sections.
The collider operates at temperatures colder than outer space for maximum efficiency and experts needed to gradually warm the damaged section to assess it.
"Now the sector is warm so they are able to go in and physically look at each of the interconnections," Mr Gillies told Associated Press.
The cost of the work will fall within the Cern's existing budget.
Dr Lyn Evans, the Welsh-born project director has called the collider "a discovery machine, the most sophisticated scientific instrument of our time."
A view of the LHC tunnel.
Map of the LHC (the large circle) and the area underneath which it lies.
The European Organization for Nuclear Research (CERN) said on Monday that repairing the Large Hadron Collider (LHC) will cost up to €16.6 million or US$21 million.

The LHC, which is the world's largest and highest-energy particle accelerator, is located near the border of France and Switzerland and crosses the border four times. It has a diameter of 27 km (17 miles). It is designed to simulate the conditions shortly after the Big Bang, but it broke down on September 19 due to an electrical failure. 

Most of the repair time is covered by previously scheduled maintenance time, and CERN originally hoped to have the machine up and running again by early May. However, CERN officials now believe that it may take until the end of July or longer.

CERN spokesman James Gillies said: "If we can do it sooner, all well and good. But I think we can do it realistically by early summer." 

The machine operates at temperatures colder than outer space and must be gradually warmed up for experts to assess the damage, causing much of the delay. CERN expects the repair cost to fall within the annual budget for the project.