The gemara (Pesachim 50b) says that in a place where they are noheg not to do Milacha (see Aruch HaShulchan what kind of Milacha is assur in this instance) on Erev Shabbos after mincha, one does Milacha will not see any Bracha from it. Nevertheless we only warn these people but we do not punish them. Although there are various opinions the Aruch HaShulchan (OC 251) says that Mincha here means Mincha Ketana which is two and a half hours (Sha'os Zmanios) before Shkia.
One wonders, says the Aruch HaShulchan how it is that many Baalei Batim work until very close to Shabbos? He is Milamed Zchus in a number of ways. a) we are meikil because of the struggle to earn a living (Docheik HaParnassa). b) The minhag only applies to business owners and not employees. c) Only starting new work is assur. d) The issur is because one must prepare for Shabbos, but in many instances the Eishes Chayil makes all the preparations.
Still, he says, a Yarei Shamayim should do like the Takanas Chachomim and it will be good for him.
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.