Reports that at least 50 girls were taken from the UK to Somalia for female genital mutilation are being investigated by Scotland Yard.
Liberal Democrat peer Baroness Tonge contacted the Metropolitan Police after spotting a large group of girls on a flight from Heathrow last Saturday.
The girls were said to be aged 11 to 17 and with their mothers or grandmothers.
It comes as Bedfordshire Police secured the UK's first FGM protection order, preventing two girls from going abroad.
The Metropolitan Police said officers from the Specialist Crime and Operations Command were investigating Lady Tonge's report.
'Scattering of grannies'
Speaking to the BBC, Lady Tonge said the girls spoke English and were of Somali origin.
"It was just odd," she said. "They were young girls and mothers and a scattering of grannies."
They were on an Ethiopian Airlines flight to Addis Ababa on 11 July and according to the Lib Dem peer transferred onto a plane to Mogadishu, the capital of Somalia.
Lady Tonge, along with the Labour MP for Halifax, Holly Lynch, was on a trip to the Financial Development Conference in Addis Ababa.
FGM, also termed female circumcision, is illegal in the UK. It refers to any procedure that alters or injures the female genital organs for non-medical reasons.
It is a painful ritual carried out on women and young girls from certain communities from Africa, Asia and the Middle East.
Lady Tonge said that both she and Ms Lynch felt the presence of so many girls at the start of the summer holidays was "suspicious", given that it was the start of the so-called "cutting season" when FGM is carried out, and she decided to raise the alarm on her return to the UK.
Scotland Yard confirmed that police had been called by a "woman concerned about a large number of girls on a flight from Heathrow to Ethiopia on 11 July whom she believed were at risk of FGM".
That confirmation from the Met came after Bedfordshire Police said it secured a protection court order on the day new powers came into effect.
The civil legislation allows officials to seize passports from people they suspect are planning on taking girls overseas for FGM, and breaching an order is a criminal offence.
The move prevents two girls being taken to Africa, Bedfordshire Police said.
The force said it is estimated that more than 20,000 girls under the age of 15 in the UK are at risk of FGM each year, although very few cases are reported.
Det Ch Insp Nick Bellingham, from Bedfordshire Police's Public Protection Unit, said: "With schools breaking up for the summer holidays today, we will continue to use this legislation where needed to prevent young girls who we believe may be at risk from being taken out of the country.
"This is child abuse, and we will do everything in our power to ensure that children are kept safe and that those responsible are caught."Aneeta Prem, founder of women's charity Freedom Charity, said the use of a protection order was a positive step.
But she warned that the authorities must also look out for "cutters" - people who carry out FGM "for as little as £200 a girl" - entering the UK.
"We can't be politically sensitive. Girls who are mutilated in this way suffer a lifetime of scarring and permanent damage," she said.
The Female Genital Mutilation Act 2003 in England, Wales and Northern Ireland, and the Prohibition of Female Genital Mutilation Act 2005 in Scotland states that FGM is illegal unless it is necessary for health reasons.
The law states that is also illegal to arrange for a UK national to be taken overseas to undergo FGM.
The order secured by Bedfordshire Police, which can be made by courts in England and Wales, was introduced under the Serious Crime Act 2015.
Equalities and Justice Minister Caroline Dinenage said the protection orders have been "fast-tracked... to make sure women and girls facing the awful threat of FGM can be kept safe".
"These orders mean girls and the communities around them now know they will have somewhere to turn, that the law is on their side and help is out there.
"The government is committed to ending FGM."
Was planeload of girls on way to Africa for FGM? Police to quiz mothers and daughters after peer raises alarm