The UK has been censured by an international committee for its failure to notify the German public of the potential environmental impacts of Hinkley Point C, the new nuclear power station being built in Somerset, south-west England.

In a political embarrassment for the government, the verdict found that the UK had not complied with the Aarhus convention, an international agreement on involving citizens in environmental matters.

The Aarhus committee’s finding followed a complaint by a German Green party politician, who said the UK should now halt work on the project to properly consult with the public in neighbouring countries.

Sylvia Kotting-Uhl told the Guardian: “If the government does not want to ridicule the trans-boundary participation procedure that is now due, construction works should be suspended until the procedure is completed.”

While the ruling is very unlikely to affect the project, which began construction last year, the breach comes on top of a fortnight of bad news for the two atomic reactors.

EDF, the French state-owned developer of Hinkley Point C, admitted this week that costs were likely to rise by £1.5bn and there was a risk of a 15-month delay, just days after the public spending watchdog called the project “risky and expensive”.

Kotting-Uhl originally complained to Aarhus’s compliance committee in the summer of 2013, before the UK had agreed commercial terms with EDF.

She said the UK government had discriminated against Germans by not giving them opportunities to take part in the environmental impact assessment procedure for Hinkley, as the Aarhus convention requires. The committee agreed.

In conclusions published recently, the committee said: “By not ensuring that the public concerned in Germany had a reasonable chance to learn about the proposed activity and the opportunities for the public to participate in the respective decision-making, the party concerned failed to comply with article 6, paragraph 2, of the convention.”

Named after the Danish city of Århus, the convention covers the involvement of the public on environmental matters relating to developments such as power stations or new roads – including access to information, the public’s participation in decision-making, and access to justice. The UK ratified the agreement in 2005.

The former UN secretary general Kofi Annan called it “the most ambitious venture in the area of environmental democracy so far undertaken under the auspices of the United Nations”.

The Aarhus committee said that ministers should put in place a legal framework to ensure that the failure to inform the public about Hinkley’s potential risks is not repeated.

In the case of nuclear power stations, it said: “Public authorities are required to consider the magnitude of the effects if an accident would indeed occur, even if the risk of an accident is very small.”

The Department of Business, Energy and Industrial Strategy said: “The government has cooperated fully with the Aarhus compliance committee investigation and there was extensive public consultation on proposals for Hinkley Point C before development consent was granted.”

The spokeswoman added that the government was confident Hinkley would have “no significant adverse effect on the environment of any other country”. The issue of notification around Hinkley is due to be discussed at a meeting in September of the 46 countries signed up to the convention.

EDF said: “We have carried out all the environmental impact assessments (EIA) required for Hinkley Point C, including assessing any likely significant transboundary impacts. In considering the EIA the UK Planning Inspectorate concluded there was no likelihood of significant transboundary effects.”