Construction of the Iter reactor will take place at Cadarache in Southern France

European Union member states have agreed the additional funds needed to construct Iter (the International Thermonuclear Experimental Reactor).

The French-based machine will prove the concept of harvesting energy from the fusion of hydrogen nuclei – the same process at the heart of the Sun.

Iter has seen its baseline price tag rise dramatically since a consortium of nations green lit the project in 2006.

The extra 1.4bn euros will cover a shortfall in building costs in 2012-13.

After months of protracted negotiations, the monies were finally sanctioned at an Agriculture and Fish Council meeting on 12 July.

The funds will come from a variety of sources within the EU budget, including from its research budget (the Framework Programme 7).

The EU's executive body, the Commission, had wanted member states to inject fresh funds into the project from outside the existing Brussels budget.

The decision paves the way for a special Council meeting of Iter itself, which will take place on 27-28 July. This will define the latest scope and schedule for the project.

Iter is a collaboration between the EU, the US, Russia, Japan, China, India and South Korea. It is the culmination of decades of research.

Iter’s tokamak will use a magnetic field to confine the plasma

Its fusion reactions will take place inside a 100-million-degree gas (plasma) suspended in an intense doughnut-shaped magnetic field.

The reactor is designed to produce 500MW of fusion power during pulses of at least 400 seconds. Critically, Iter is expected to demonstrate the principle that it possible to get far more energy out of the process than is used to initiate it.

The original plan was to build the experiment within 10 years for a budget of 5bn euros. But a range of issues, from technical to personnel matters, have conspired to inflate Iter's final price.

Many now expect it to be in the region of 15bn euros; and the total cost of construction for the EU – a major partner in Iter – is put at no less than 7.2bn euros.

EU ministers had tried – and failed – to resolve arguments over where the extra immediate funds should come from at their 26 May Competitiveness Council.

A task force was then established to find a solution to the issue.

It has been agreed that the additional 1.4bn euros required for 2012-2013 will be taken from a mix of sources within the current EU budget, including from the Framework Programme.

Many scientists have expressed the fear that raiding the 53bn-euro FP7 pot could damage other projects.

At the European Science Open Forum last week in Turin, Italy, Prof Helga Nowotny, the new president of the European Research Council, again voiced her concern at the impact rising Iter costs could have on other types of European scientific activity.

A spokesperson for the UK government told BBC News: "We are pleased to note the conclusions call for important improvements to the financial management and oversight of the project.

"We agree with the view expressed in the conclusions that the additional financial commitments for ITER for 2012 and 2013 should come from a mix of sources within the current EU budget 2007-2013."