Scott Wilson, who helped lead the response to the 2017 London Bridge attack, warned police could be left monitoring ISIS supporters “forever” if they return to the UK.
Begum, who left her east London home to join ISIS in Syria in 2015, wants to return to the UK for the sake of her newborn baby boy Jarrah.
She has been stripped of her British citizenship – but this is being challenged is it is illegal to leave someone “stateless”.
The ISIS bride initially showed no remorse and attempted to justify the Manchester Arena bombing.
But since she was uncovered in the al-Hawz refugee camp in Syria, Begum has said she will be able to change.
“If you make it easy then you’re just opening the floodgates for it ”
Mr Wilson, who was UK National Counter Terrorism Co-ordinator for Protect and Prepare until last summer, warned a slew of ISIS supporters will want to return to the UK if Begum succeeds in doing so.
He said: “I don’t think the British Government should give her the way to get her out of Syria and get her back here, that’s the point,” he said. “If she gets back here then it’s on their plate to deal with it.
“If you start making a free way for them to get back then where do you stop?
“I can see floodgates opening. I don’t think we should make it easy. If you make it easy then you’re just opening the floodgates for it and suddenly you’re monitoring people forever.
“I don’t sound very sympathetic because I just see the consequences. I dealt with London Bridge on the streets.
“When you see the consequences of this, I just think this country can be too sympathetic at times. She made the decision so she will have to stick to the decision she made.”
The top cop added; “When you look at all the background to her going out there, I know they are trying to say she was a groomed child, but there was a lot of planning that went in with her and her group to get herself out there.
“It’s not just the bringing her back, the deradicalisation programme; the security services will end up spending years monitoring her.
“They will never be able to take their eye off her for the simple reason that they don’t know what could happen. If she did do something in the UK, the Government and the security services would get the blame for it.
“It’s the millions of pounds of monitoring that it’s going to cause, not just the bringing her back and putting her through a deradicalisation programme.”
Last week, Oxford-born terror suspect Jack Letts, known as Jihadi Jack, said he wants to return to the UK, and misses his mother, pasties and Doctor Who.
It was also reported that Abu Hamza’s son, Sufyan Mustafa, 23, is fighting the decision to revoke his British passport.
Scores of British fighters are believed to remain in the Middle East have fled the UK to join ISIS.
Begum has reignited the debate about what will happen to them once ISIS is utterly defeated in Syria.
Begum’s father, Ahmed Ali, said his 19-year-old daughter should face justice in Britain despite previously appearing to back the removal of her citizenship.
The teenager, who married Dutch ISIS fighter Yago Riedijk, had two older children who both died.
Mr Ali told ITV News: “I don’t think he’s (Sajid Javid) done the right thing because she is a British citizen, and if it turns out she has committed any crimes, then she should face justice in the UK.
“She belongs to this country, she belongs to England.
“My daughter was a little child, she made a mistake, she didn’t properly understand.”