Half of all migrants claiming to be children are lying, according to Austrian officials who say the 'juveniles' are sometimes in their 30s and have grey hair and beards.

The unnamed official told Austrian journalists: 'It is completely absurd. We have men turning up here with full beards and grey hair claiming to be children.'

Austrian officials said that of 2,200 asylum seekers claiming to be children, 951 were found to be considerably older following medical tests.

Migrants who have crossed the border from Austria are pictured walking to a registration point in the village of Simbach in southern Germany. Officials in Austria say half of all migrants who say they are children are adults

But under Austrian law every person claiming to be a juvenile has to be given child status, until medical tests can prove otherwise.

An official from Austria's Bundesamt für Fremdenwesen und Asyl (BFA) immigration agency said two million euros (£1.59m) had been spent on these medical tests.

Very few migrants from Syria, Afghanistan or Nigeria have birth certificates or other identity papers which can be relied upon to give an accurate estimate of their age.

One police officer working at the Traiskirchen refugee centre, south of Vienna, told the Kronen Zeitung newspaper: 'We have fully grown men with beards and grey hair who are telling us that they are 17 years old.

He added: 'When they tell us that they are children, it instantly gives them the right to have their family join them as they are children. It also gives them better conditions and legal rights especially with regard to their asylum applications.

'We have fully grown men with beards and grey hair who are telling us that they are 17 years old Police officer

'Even when it's completely obvious that the person in front of us is not 17 years old, we are forced to treat them as a child until medical tests prove otherwise,' he added.

A spokesman for Interior Minister Karl-Heinz Grundboeck said: 'If there is a suggestion that a person claiming to be a child is not the age they claim, then a test is organised. That means a medical examination and a professional report by a qualified expert.'

The number of fake claims has shot up.

In 2014 there was only one bogus child claim out of 178, whereas last year there were 951 out of 2,200.

The majority of those 'fake children' were from Afghanistan (691), with only 46 from Somalia, 44 from Nigeria and 40 from Pakistan.

A migrant walks to a refugee camp on the Greece/Macedonia border. The migrant crisis has resulted in huge numbers of asylum application in Austria, many of which are from people who are claiming to be children

In a small number of cases the medical test cannot prove adulthood.

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Around 1,000 child migrants were reported to have gone missing in Greece but it appears that many of them may have actually been adults who managed to get across the border and tried their luck in Hungary, Austria or Bulgaria.

A migrant's age can have a far-reaching impact on their lives.

In 2005 a woman was shot dead while holding a baby at a party in Peckham, south London.

Her killers Timy and Diamond Babamuboni claimed to be 15 and 17 at the time but police had severe doubts and both had forged Nigerian birth certificates.

Police had wanted to carry out dental tests which would have identified their age but the pair both refused to co-operate.