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Immigration authorities are seriously concerned over a huge rise in the number of Portuguese women taking part in "marriages of convenience" here.

The sham marriages are being used by nationals from non-EEA countries (the 28 EU member states plus Iceland, Norway and Lichtenstein) to ensure they can live here legally.

Most of the men involved are from India and Pakistan and have outstayed their visas.

In the past, the immigration authorities successfully tackled a major problem with sham marriages when the women were being supplied from Latvia. However, recent monitoring shows that the Portuguese are now filling the gaps created by the departure of the Latvian supply gangs.

The number of Portuguese women involved has jumped almost four-fold in the past four years, up from 80 in 2011 to almost 300 so far this year.

Most of the women are in their mid-20s.

Officials say they are not being trafficked here but come from impoverished backgrounds and see an opportunity to make a financial "killing".

It is believed that sums of more than €5,000 have changed hands in some of the deals.

Most of the women return to Portugal within days of the transaction, one official told the Irish Independent.

The Department of Justice would not comment on the extent of the shams but said the Immigration Service continually monitored trends in that area and had noticed a marked increase in marriages between Portuguese and non-EEA nationals taking place in Ireland over the past year.

Talks are currently being held with the Portuguese authorities to help combat the abuse.

Officers from the Garda national immigration bureau also carry out regular investigations into suspected marriages of convenience.

The department said it was amending the free movement regulations, which help implement the EU free movement directive, to make it harder for the participants to "derive an immigra tion benefit".

But it acknowledged that this was a complex area.

Investigation

"Marriage is constitutionally protected in Ireland and this makes if very difficult to criminalise the act of marriage, notwithstanding the motivation behind it," one official said.

"Therefore, the best means of combating it comprises a combination of legislation, intensive investigation and co-operation across EU governments."

A recent High Court judgment has also restricted the measures that could have been taken by the gardai in the past.

In 2001, there were 20 applications here for residency based on marriage to an EU national. But the total increased to 2,700 within nine years.

An investigation into marriage patterns in 2010 found applications for residency based on marriage to a Latvian spouse from 116 Pakistani nationals, 13 Nigerians and 36 Indian nationals.

Irish Independent