A significant number of sham marriages may be going undetected as register offices fail to report suspicions to immigration officials, an inspection has found.

While observing an operation by West London enforcement staff, borders and immigration inspectors were told suspicions about sham marriage are being under-reported by register offices, despite there being a statutory duty on registrars to refer such marriages to the Home Office.

The Home Office should work with the General Register Office to ensure that all suspicions are reported, chief inspector of borders and immigration John Vine said.

In most sham marriages, a national of a country outside the European Economic Area (EEA) marries an EEA national in the UK. If not detected, they allow individuals to obtain residency for five years and then settle permanently in the UK on the basis of relationships that are not genuine.

Mr Vine said: "Many register offices refer few, if any, cases of suspected sham marriage to the Home Office, despite the fact that they have a statutory duty to do so.

"This means that a significant number of sham marriages may be going undetected.

"It also creates a risk that sham marriages will be displaced from areas where they are identified to those where they are not.

"The Home Office must work with the General Register Office to ensure that all such marriages are referred to it by local register offices in order that a true picture of this problem can be painted."

Mr Vine and his team looked at an enforcement operation at Brent Register Office in north west London, under Operation Mellor, the Home Office's initiative to tackle sham marriages and sham civil partnerships, which began on January 14 last year.

The enforcement operation at Brent was triggered by reported suspicions about two couples due to be married on the morning of the inspection.

Between January 14 and September 30 last year, the Home Office carried out 500 Mellor enforcement operations, leading to 334 arrests for immigration offences. By September 30, 78 people had been removed from the UK.

Looking at the national picture, Home Office staff and managers told inspectors that sham marriage was a "growing problem" but added that knowledge of the extent of the problem and the intelligence to deal with it was "lagging behind".

Brent Register Office told inspectors that it considered it would have enough sham marriage and immigration offences work for an enforcement officer to be stationed there every day.

But Mr Vine warned that more reports from register offices and the Home Office's increasing focus on tackling sham marriage will pile pressure on the 19 enforcement teams around the country as they will not have enough resources.

He said: "The Home Office should therefore ensure that local enforcement teams are adequately resourced to deal with any increase in referrals of suspected sham marriages."

The Home Office also accepted there were intelligence gaps in relation to setting up sham marriages and other linked criminal activity, the inspector's report said.

There is no requirement for churches to report any suspicions about marriages, but some staff believed sham marriage was a problem in church weddings, he added.

In addition, a national target for sham marriage operations has been set by the Home Office but there are no local targets.

Following the Brent operation, two immigration offenders were identified and were taken back to the west London offices in a celled van and transferred to a detention facility.

Removal arrangements were made for both, but these had to be dropped as they sought and were granted permission to seek a judicial review of their removals on Human Rights Act grounds, the inspector said.

The two were detained for a combined total of 51 nights before being released on bail, which cost more than £6,000, he added.

On release, they were placed under reporting conditions pending the outcomes of their judicial reviews.

Brent is one of the most ethnically-diverse London boroughs.

According to the 2011 census, Brent's total population was 311,215, of whom 216,277 or 69% were black, Asian or from other minority ethnic groups.

More than 55% of the total population in Brent was born outside the UK.

Brent Civic Centre, where the enforcement operation was observed, conducts around 1,500 marriages and 30 civil partnerships each year.

The Home Office has recently recruited 400 staff in frontline enforcement teams.

A spokeswoman said: "The Government is already taking action to crack down on those who abuse the marriage system in an attempt to cheat our immigration system. And we are determined to do more.

"Registrars have a duty to report suspected sham marriages to the Home Office and we are working more closely with the General Register Office to increase awareness and improve the national response.

"The Immigration Bill will introduce new measures to give our officers and registrars more time to investigate, prosecute and remove those attempting to stage sham marriages."