The number of reports of theft on the London Underground has increased by more than 80% in the past three years.
The figures from British Transport Police (BTP) showed in 2016-17 there were 3,730 reports. This number increased to 6,825 in 2018-19.
The Piccadilly and Central lines saw the highest number of thefts, with a combined total of 2,250 reports made in the past year.
BTP said it was working hard to tackle theft.
The figures released to BBC London covered theft of passenger property, which included bag thefts and pickpocketing.
Steve, a BTP undercover officer, who did not want his full name used because of his undercover work, said: "It's got a lot busier lately, a lot busier. We're arresting a lot more people.
"In 14 years of doing pickpockets, I've never known it be this busy."
Steve and other undercover teams work across the Underground looking out for thefts.
He said they could arrest perpetrators if they saw a theft being attempted and their aim was to stop it before it happened.
However, he said getting into the right position to see a crime happen unnoticed was difficult as some were carried out by very well-organised groups.
He said he was not sure why there had been an increase, but said some people he had arrested who had entered the country from Europe to pickpocket had told him it was a "last chance saloon" because of Brexit.
Det Insp Bob Stokoe, head of theft of passenger property at BTP, said: "I don't think we can deny that it is incredibly disappointing that the numbers have gone up but the London Underground is still an incredibly safe environment.
"There are in excess of five million passenger journeys a day so it's four crimes of theft for every million journeys."
He said a new text report system meant it was now easier to report a theft, which may have increased the number of reports.