Image caption Mathew Bell directed the attacks from his home in Irvine

British authorities failed to arrest a paedophile for 18 months after a tip-off about him, the BBC has learned.

The National Crime Agency was first warned about the activities of Matthew Bell, 51, in September 2016 - but he was not arrested until March 2018.

Bell continued to pay to watch the abuse of Filipino children until at least April 2017, court papers show.

The NCA said there was not enough evidence to arrest him initially, but it "acted swiftly" when more emerged.

However, the chairwoman of the House of Commons home affairs committee said the case was "deeply worrying".

Matthew Bell, from Irvine, North Ayrshire, is thought to be the first man in Scotland to be convicted of live streaming the sexual abuse of children.

Earlier this month he pleaded guilty to five offences - the judge describing his crimes as being "of the utmost depravity".

Bell would pay as little as 93p to watch on a webcam from his home as children in the Philippines were forced to carry out sex acts.

But we can now reveal the NCA was first warned about Bell in September 2016 - 18 months before he was eventually arrested.

The NCA says it didn't have enough information to make an arrest in 2016, but after BBC News brought forward extra material officers were able to "develop intelligence" about the case and "act quickly".

Image caption Investigative journalist Peter Dupont took this picture of Bell on his webcam

This story starts not in Scotland but a small town near Brussels, Belgium, and a man called Peter Dupont.

He is an investigative journalist who wanted to find out more about the live-streaming gangs in the Philippines who abuse children for the benefit of Western paedophiles watching via webcam.

It's a huge and growing problem. Children, even babies, are being sexually exploited for cash often by their own families.

So Mr Dupont went undercover and infiltrated a group based in Iligan in the south of the Philippines.

Mr Dupont was called by an unknown Skype number and stumbled into a group Skype session.

On screen was Bell, sitting in his flat, 25 miles from Glasgow, directing the sexual abuse of an 11-year-old girl.

"That was one of the most gruesome things - he was enjoying it very much, he was laughing the whole time," Mr Dupont told the BBC.

Horrified, the journalist took several screenshots of Bell and dropped the call.

Image caption Mr Dupont first contacted the Child Exploitation and Online Protection command in 2016

Mr Dupont continued his work, gathering evidence on numerous foreign paedophiles. He was also writing a book and making a documentary with the help of two non-governmental organisations (NGOs).

In April 2015, he took all the information he gathered to the police in the Philippines. Five adults were arrested in Iligan and 12 children were rescued.

He continued to work with the Philippine National Police and a US charity called the International Justice Mission. But he wanted Bell to face justice too.

In January 2016 Mr Dupont gave an interview to the Daily Mail about his investigation - a picture of Bell only partially disguised appeared in the paper.

The journalist said he expected British police to get in touch - but they didn't. From then, he tried to make contact with the British authorities. Here is a timeline:

July 2016: Mr Dupont contacts the office of the Child Exploitation and Online Protection command of Britain's National Crime Agency, Ceop, in Thailand. No one responds

20 September 2016: Officer at Interpol tells him "the UK police is strongly determined to investigate" and he is directed to Ceop in London

21 September 2016: Email from a Ceop officer in London asking for information on "the individual whose photo appeared in the Daily Mail"

23 September 2016: Mr Dupont sends a screenshot of Bell's Skype account, two pictures of him, his date of birth, his skype ID and tells them his home city is Glasgow

November 2016: Mr Dupont says a Ceop officer told him he wanted to travel to Brussels to interview him "ASAP". The journalist says "I never heard from him again". The NCA disputes that version of events

15 November 2016: Mr Dupont speaks at an Interpol conference about crimes against children

March 2017: Ceop asks him for information on other "possible British offenders". He again sends Bell's Skype name and email address

13 April 2017: Ceop says it "cannot immediately identify the Scottish man named Matthew"

3 May 2017: Mr Dupont sends Ceop a statement about how he found Bell, along with email addresses, skype ID, pictures, YouTube videos and Bell's date of birth. He never hears from Ceop again

In January 2018, the BBC heard about the case and travelled to Belgium to interview Mr Dupont.

He did not understand why no action had been taken against Bell. "It's a huge shame," he told us. "It's pure negligence."

A month later we took all the information we had gathered to Ceop, and on 21 March 2018 Bell was arrested.

That arrest took place 18 months after Mr Dupont says he first passed enough information to identify Bell to authorities.

On 2 July this year, Bell pleaded guilty to five offences. However, court documents show that he was still abusing children in April 2017, more than a year after his image appeared in the Daily Mail and seven months after Ceop's initial tip-off.

Image copyright Reuters Image caption Labour MP Yvette Cooper said the case was "really worrying"

The NCA said: "In September 2016 we received information from Mr Dupont which, despite researching and developing, provided insufficient evidence for action to be taken against Bell and the case remained open.

"We thank the BBC for their visit in February 2018, after which we continued to develop intelligence enabling us to act swiftly in partnership with Police Scotland to arrest and convict Bell."

It said an internal review found there was "no referable or recordable conduct" and no need to involve the Police Investigations and Review Commissioner, which investigates serious incidents involving the police.

But questions about this case and the wider issues facing the NCA are now being asked at Westminster.

Labour MP Yvette Cooper, who is chair of the Home Affairs Select Committee, described the case as "incredibly disturbing".

"To have delays in a case like this, which is so serious with such a vile crime against children, is really worrying."

She added: "I've been concerned for many years that there just aren't enough resources going into this given the scale of the escalating problem we face."

Bell will be sentenced next week, but questions remain for the NCA about why it took so long to catch him - and how many children were left at risk.