New headache for David Cameron, as senior adviser to the European Court of Justice rejects Britain's blanket policy of deporting foreign criminals

Foreign criminals cannot be automatically deported because it would breach their children's rights to live as EU citizens, the European Court of Justice looks set to rule, in what would be a significant blow to Theresa May's Home Office.

The advocate general of Europe’s highest court said that foreign offenders whose children have British nationality cannot be expelled “simply” because they have committed a crime.

If the court’s final ruling backs this preliminary opinion, it would overturn British laws that say the Home Secretary is obliged to automatically deport all non-EU foreigners who are sentenced to more than one year in prison, regardless of their family circumstances.

nstead, the Home Secretary must weigh up in each case whether the criminal presents a “genuine, present and sufficiently serious” threat to society. That includes judging their health, economic circumstances and likelihood of reoffending, the court said.

It would open the door to potentially hundreds of low-level foreign criminals claiming the right to stay in Britain.

The case intensifies pressure on David Cameron, whose renegotiation has not touched on the powers of the ECJ over British law. Yesterday he promised to bring forward legislative proposals soon that will “put beyond doubt” the sovereignty of the Commons over European law.

Mrs May, a frequent critic of free movement, has hinted in recent days that she will campaign to remain in the European Union.

The case relates to a Moroccan woman, known as CS, who is the sole carer for her son by a British husband, who holds British citizenship.

She was jailed for 12 months in 2012 and after completing her sentence was handed a deportation notice.

CS appealed, and the case appeared before an immigration tribunal, who agreed that it amount to the “constructive expulsion” of her son from the EU and therefore violate his rights as a European citizen. Mrs May appealed against that decision, and the case was sent to the Luxembourg court.

The final fate of CS has not yet been decided by the court.

But in his legal advice which gives a strong guide as to the verdict, the advocate general, Maciej Szpunar, declared the British blanket deportation rule is “in principle” a violation of EU laws if it would “deprive the child who is a citizen of the Union of genuine enjoyment of the substance of his or her rights.”

However, deporting parents such as CS can be permitted in “exceptional circumstances”, after the government has examined the “personal conduct” of the criminal and assessed whether they present a serious risk to “public security”.

Yesterday, the Prime Minister confirmed plans to bring forward a Bill of Rights. “Asserting the sovereignty of this House, that is something we did in 2011 through our European Union Act. It's something I am keen to do even more of, to put beyond doubt that this House of Commons is sovereign and that is something we will look to do at the same time as concluding these negotiations.”

However, Michael Gove, the Justice Secretary admitted that the forthcoming Bill of Rights will “still be subject to the primacy of European law”.

Critics will say that the new law is only symbolic and will have no practical consequences, given that defying ECJ rulings is ultimately incompatible with EU membership.

Earlier this week, Mr Szpunar presented another headache for Britain, when he argued that France had no right under EU directives to jail a Ghanaian woman who had attempted to reach London via the Channel Tunnel using someone else's passport.

European court challenges Britain's right to deport all foreign criminals