Neil Patrick Harris has quashed rumors Britney Spears will be a regular castmember on his TV show How I Met Your Mother.

The Toxic star gained critical acclaim - and helped the hit U.s. TV series score its best ever ratings - when she played a guest role as a secretary in a episode which was aired last month.

But Harris insists reports the singer has secured a permanent spot on the show after impressing bosses are untrue.

He says, "Well, uh, no. (They're) totally unfounded rumors."

And Harris insists Spears isn't ready to become a full-time actress: "She's not an actress. She played a role for a week, and did a really nice job, but the workload involved with doing memorization and the purging of comedy over and over is really, really hard. I mean, it takes a strong skill set to do that every week. You do it one week, it's fine, but to do it a lot of weeks is hard. She (Spears) is not going to be a regular on our show."