Germany's top court began hearing a lawsuit Tuesday against mass property grabs to make way for an enormous coal mine with broad implications for the country's historic "energy transformation" project.

The Federal Constitutional Court is examining complaints by affected homeowners and an environmentalist group against the Garzweiler II mine, which threatens to swallow several villages.

Around 7,600 people will be asked to move to accommodate the gigantic project in North Rhine-Westphalia, Germany's most populous state, which comes as the country steps up the use of coal to compensate for a complete phase-out of nuclear power.

The highly anticipated decision by the court's scarlet-robed judges, expected later this year, could have wide-reaching implications for Germany's energy mix.

A ruling in favour of the plaintiffs could trigger an overhaul of laws governing mining practices and property rights.

The Garzweiler II mine near the city of Moenchengladbach is to measure a staggering 48 square kilometres (19 square miles), slightly smaller than the area of Manhattan.

It is believed to hold 1.3 billion tonnes of lignite or brown coal, to be extracted by energy company RWE over the next three decades and whose burning will belch 1.2 billion tonnes of the greenhouse gas carbon dioxide into the air.

Garzweiler II is an expansion of an existing mine which, taken together, would cover 112 square kilometres. Work on the extension began in 2006.

Under current German law, compulsory expropriation of property is allowed if the exploitation of the raw materials on the site "serves the greater good".

However an earlier ruling by the federal court found that such steps were only acceptable when there was "a particularly urgent public interest" in the project.

Germany, Europe's top economy, decided after Japan's 2011 Fukushima nuclear accident to phase out nuclear power by 2022, an about-face that started with the immediate closure of the eight oldest plants.

Since then Germany has accelerated a boom in wind farms, solar power and biofuels, promoted by subsidies and legal reforms, with the goal of generating half of its electricity from renewables by 2030.

But the move has in the short term increased the country's reliance on the heavy polluter lignite, or brown coal, the source of about one-quarter of Germany's electricity supply.

Much of the energy produced is sold abroad, according to Dirk Jansen of the ecologist group BUND, one of the plaintiffs.

He argued that Germany's energy needs would be better served with more natural gas plants rather than coal.

The argument was backed up by a recent study by the influential economic institute DIW indicating that brown coal was not essential to meeting Germany's electricity needs under the energy transformation.