Kevin Federline Says Britney Spears' Behavior Is Alarming to Kids, Family

Kevin Federline says Britney Spears is raising tons of red flags since her conservatorship ended ... and he says her sons and family are alarmed with what they're seeing from her.

Britney's ex-husband joined us on "TMZ Live" Wednesday and we asked him if he thought it was a mistake to end the conservatorship.

Kevin says everyone in the family -- including Britney's two sons Sean Preston and Jayden James -- are concerned for her well-being and he says his new memoir, "You Thought You Knew," is a cry for help to gain support for Britney and their kids.

K-Fed says folks need to read his book to truly understand his thoughts on the conservatorship ... but he says the boys want their mom to be around for years to come and want a relationship with her.

Kevin says the family's been vilified for their roles in the conservatorship, which he called a "difficult situation," but he says everyone is united in wanting Britney to be happy and healthy.

Speaking of red flags, Kevin's memoir recalls one that popped up the night before their wedding ... when he caught Britney on the phone with her ex, Justin Timberlake. Kevin takes us back in time to that moment and we ask him what was going through his mind ... and he explains why they went through with their nuptials.

He also dishes in the memoir about the time he walked in on Britney and another woman in Amsterdam.

We've been covering Kevin and Britney for 20 years at TMZ and here he sounds more mature than ever before ... ya gotta watch the full interview on "TMZ Live" to see how he responds to some serious shade thrown his way by Britney and another of her exes, Sam Asghari.