"Maggie loves her work and her performances in Downton have helped make it a worldwide success.
"But she will have to think very carefully now about whether she stays in the show. Her health always comes first and I am sure she will take advice from the experts."
The source continued: "Maggie was in the best possible hands at the Wellington Hospital.
"It was obviously a worrying time for her but she underwent all the tests possible. She was only discharged when the cardiologist was completely satisfied she was OK."
'SHE IS FINE AND WELL'
However, a rep for Smith has since played down reports the actress suffered a health scare.
The actress's spokesperson told the Daily Mail: "She is fine and well and at home."
According to the Mail, the reports emerged after Smith visited her doctor for a routine medical procedure.
Meanwhile, Downton Abbey creator Julian Fellowes says that another series of the period drama is likely.
The writer told (Potential spoilers in the following link, NZ readers) The Telegraph that he would be "surprised" if a fourth series wasn't commissioned.
"I would hope [there will be more] because of the response to this series, but you know [ITV] are always pretty close to their chest in terms of an actual commission," he said. "But I think I'd be surprised if there was not a fourth series."
Fellowes said he was pleased with the reception of the third series.
"I am pleased with this series. I thought it worked well. I think it is an interesting time, that funny time after the Great War when people didn't quite know how much everything had gone back to the way it was before or how much it had changed. They were sort of feeling their way towards the realisation at the end of the 20s that things had changed enormously. It is a funny sort of insecure period and it is not one that is much done.
"You have lots of stuff about the Edwardians and everyone sort of jumps forward to the 30s and the Nazis. So I was pleased we were allowed to be in that time.
"It seems to me that the actors have made a very good job of it and the audience response has been great and they kept watching," he said.
Fellowes also claimed that the show's second series was "more solid" than the first.
"For me I felt the second [series] was more solid than the first because we knew who they all were. And now we are really involved with them and that takes you forward another step.
"I am pleased with all of them really. It sounds rather smug, but I have been very happy with them," he said.
The screenwriter also dismissed reports that Smith was unwell.
"She's absolutely fine and going out for lunch," he said.
Earlier this year, Fellowes spoke about Smith's longevity in the show.
"Dame Maggie will be in this show for as long as she wants to be and I can tell you that even if she doesn't want to be, she won't die, she'll go to the seaside just in case she wants to come back."
He added: "With Maggie, you never know. Others sign on for the next series and they sign for this and they sign for that, but she doesn't. She stays her own person. But as far as I'm concerned I hope she stays with the show until Downton falls down."
Asked how long she'll remain in the show, Smith told USA TODAY in September: "Listen, I must be 110 by now. Granny is going to kick the bucket at some point."
The third season of Downton Abbey concluded in the UK on Sunday and drew a season-high audience for the costume drama, according to The Hollywood Reporter.
FAST GOSSIP
And finally...
* You read it here first: Britney Spears in talks to write novel.
* Mugshot of the day: Avatar's Sam Worthington doesn't look like a happy camper after arrest for disorderly conduct.
* Fans get a first look at Brad Pitt in zombie-packed World War Z clip.
* Channing Tatum tipped to be named People's Sexiest Man Alive next week.
* Newlyweds Evan Rachel Wood and Jamie Bell robbed.
* Kirstie Alley claims intense attraction to Patrick Swayze, days after calling John Travolta the "love of my life".
* Harrison Ford open to reprising Star Wars Han Solo role.
* Shia LaBeouf denies demanding $18m to star in Transformers 4.
* Chris Brown coming between Katy Perry and Rihanna?
* It's (almost) tinsel time: The Backstreet Boys debut new Christmas song.
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