EU citizens without settled status could be subject to Home Office “sham marriage” investigations, internal guidance has revealed.

The National has seen a 45-page document of advice for Immigration Compliance and Enforcement (ICE) officers dedicated solely to “marriage investigations” ahead of the scheme closing on June 30.

Campaigners have hit out at the move as another example of the UK’s “hostile environment”, while the SNP have called for the settlement scheme deadline to be extended.

A sham marriage, or marriage of convenience, applies to couples where one person isn’t a “relevant national” from the UK, Ireland, or with confirmed or pending status on the EU settlement scheme.

It also applies to cases where there is “no genuine relationship” between the couple intending to marry, or if they enter into the marriage for “the purpose of circumventing UK immigration controls”.

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The guidance reads: “EEA citizens who do not have status under the EUSS [EU Settlement Scheme] or a decision pending on an EUSS application submitted before July 1, 2021, are within scope of the sham marriage referral and investigation scheme.”

British and Irish citizens are exempt from the investigations, and any EEA citizens without settled status – if they live elsewhere – will have to get a mandatory marriage visa if they want to travel to the UK to marry.

The guidance also goes into detail on how to prosecute any infringements, as although marriages are not illegal, there are other criminal offences which EEA citizens could be charged with.

These include facilitation (assisting people to enter the UK illegally), conspiracy to facilitate, deception, bigamy, perjury, and securing or seeking to secure the avoidance, postponement or revocation of enforcement action.

And there are scores of pages of detailed information on how ICE officers should conduct their investigations, interviews with “subjects” and where to record the information.

Although it notes that unannounced marriage investigation visits to “test the genuineness of the relationship” should not be made, it leaves a caveat for enforcers.

It reads: “Where information suggests that a sham or forced marriage has been planned or has already been contracted it may be appropriate to make enquiries at the residential address in order to assess whether an offence or breach has been committed.”

It adds that if no-one is home, officers can speak to neighbours to find out “who lives at the residential address” and at least two visits must be made.

It even goes into detail about what EEA citizens could be facing in an interview over a perceived sham marriage – such as the background and “subsistence” of the relationship, their general background and immigration history, living arrangements, arrangements for the marriage or civil partnership and the future plans of the couple.

Chillingly, the guidance adds: “The type of questions you ask will differ depending on the type of marriage and the cultural backgrounds of the couple.”

Campaigners and politicians have criticised the tight deadline for the settlement scheme and called for a review of the procedures that will affect those who do not apply in time.

A spokesperson from the3million, a grassroots organisation made up of EU citizens living in the UK, said: “This is another example of, from July 1, EU citizens facing the impact and consequences of the UK’s hostile environment.

“Wendy Williams, in her Windrush Lessons Learned Review called on a full review of the hostile environment policy and measures – ‘including assessing whether they are effective and proportionate in meeting their stated aim’.”

SNP MP and shadow home secretary Stuart McDonald said: “It is essential that the UK Government extends the deadline for the EU Settlement Scheme. Many people have still not applied for the scheme and more time is needed so eligible citizens can avoid the life-changing consequences of missing the deadline.

“The risks for EU citizens who do not apply in time are significant – preventing them from working, studying, claiming benefits, driving a car or opening a bank account. It would turn many people’s lives inside out. Brexit has already hit Scotland hard – costing us billions of pounds and diminishing our rights. The only way to keep Scotland safe from the long-term damage of Brexit is to become an independent country.”

Future Borders and Immigration Minister Kevin Foster said that there had been five million EU citizens granted settled status so far and the Government has given £22 million to support organisations helping people sign up to the scheme.

He said: “This funding is available up until the end of September to help our grant funded network continue their vital work.

“We have published guidance on grounds for making a late application and we will take a pragmatic and flexible approach when considering these applications.”