The union for senior civil servants has told the High Court its members could be made to break the law, under the government’s plan to send some asylum seekers to Rwanda.

Launching an unprecedented challenge, the FDA urged the court to clarify whether ministers could tell staff to ignore a European Court of Human Rights order to stop a flight.

The union says Civil Service rules mean they must abide by all laws, including orders from the Strasbourg-based court.

The outcome of the case could raise complex questions about the powers of ministers, with significant constitutional implications.

Earlier this week, the government confirmed to the High Court there would be no Rwanda flight before 24 July. This case must be settled before then.

Labour says it will abolish the scheme if it wins the general election on 4 July.

In June 2022, the government’s first planned Rwanda flight was aborted, after a European Court of Human Rights (ECtHR) interim order said the asylum seekers on board should not be taken to the African nation until British judges had ruled on whether they would be humanely treated.

That judgement came last November, when the UK Supreme Court ruled Rwanda was not safe for asylum seekers.

And UK Prime Minister Rishi Sunak then introduced an alternative plan, which tells judges to ignore those concerns – and says ministers could ignore a future flight-stopping ECtHR order.

Tom Hickman KC, for the union, told the High Court senior civil servants did not oppose the Rwanda scheme, as their duty was to serve the government of the day.

But the new version of the plan had left them in an unclear and potentially unlawful position.

The Civil Service code required officials to abide by domestic laws – those passed by Parliament – and international laws the UK had joined, including the European Convention on Human Rights, which the ECtHR oversees.

The government could have rewritten the Civil Service code to tell officials to ignore interim ECtHR rulings but had not done so.

“International law is no less important than domestic law,” Mr Hickman said.

“The UK is a fully subscribed member of the international-law club – and international law is fundamental to the stability of the international community.

“They [ministers] want to refuse to implement an interim measure [of the European Court of Human Rights] while wanting to maintain the appearance that international law is being complied with.”

But the government says the union is wrong.

In written submissions, its top barrister Sir James Eadie KC said if the FDA won the case, it would mean civil servants could override ministers’ decisions.

“That is constitutionally untenable,” he said.

“It would frustrate and undermine the intention of Parliament [in passing the Rwanda plan].”

Mr Justice Chamberlain said he would give his decision at a later date.