Starmer said protecting time to spend with his son and daughter made him more relaxed and a better decision-maker.
The Labour leader and his wife Victoria have a 16-year-old son and a 13-year-old daughter.
Incumbent UK Prime Minister Rishi Sunak says he has never finished work by 6pm. (AP PHOTO)
He told Virgin Radio: "We've had a strategy in place and we'll try to keep to it, which is to carve out really protected time for the kids, so on a Friday - I've been doing this for years - I will not do a work-related thing after six o'clock, pretty well come what may.
"There are a few exceptions, but that's what we do."
But Rishi Sunak took a swipe at the Labour leader, telling reporters: "I haven't finished at six ever."
And a Conservative Party attack on social media said: "Keir Starmer has said he'd clock off work at 6pm if he became prime minister.
"You deserve better than a part-time prime minister. The only way to prevent this is to vote Conservative on Thursday."
Starmer said his son and daughter are his "pride and joy" and "I don't want to lose that time".
He said that in politics "some people think, if you fill your diary 24/7 and don't do anything else, that makes you a much better decision-maker".
Britain goes to the polls on Thursday, with Labour expected to win in a landslide. (EPA PHOTO)
"I don't agree with that, I think you've got to make space, so we do it."
In an interview with Chris Evans, who used to play football with the Labour leader, Starmer suggested his children appeared unimpressed with his political career.
After winning a Spectator politician of the year award, he said his son asked "How did you blag that, then?"
And when he was speaking at a fundraising dinner his daughter asked: "Why would anyone pay to hear you speak?"
"They keep me very, very grounded," Starmer said.
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