The owners of Darley Mill Centre have reported a 42 per cent drop in sales last week following the closure of the B6451.
The road closure took effect on March 31 and is set to last between six and eight weeks.
According to North Yorkshire County Council (NYCC), the closure is a result of the necessary work taking place to ensure Long Slack Bridge can continue to hold vehicles.
Co-owner of the Darley Mill Centre Kirsteen Leventhal said: “Basically we are getting no customers, that is the problem. We were dead last week.
“While I understand the importance of maintaining roads, there are ways and means of dealing with these things.”
She added: “One of our busiest periods is Easter, and no due consideration was given to the impact this would have on the local community.
“I can’t afford to lose money like this for eight weeks.”
Though there is a signposted diversion from the A59 to the mill centre via Killinghall and Ripley and into Summerbridge, this adds an extra 15.6miles to the usual journey.
The mill centre management said that this could be avoided if visitors were diverted two miles along Cold Cotes Lane and Menwith Hill Road.
An NYCC spokesperson said: “When closing a road we have to almost always divert traffic on to a road of equivalent class. The exception would be when a road that has traffic diverted onto it has no significant safety issues and is to an equivalent standard as the road that has been closed.
“Unfortunately in this instance, this has meant the signed legal diversion route being of a significant length. We are aware that there are local ‘short cuts’ around Menwith Hill, however these cannot be offered as an official signed diversion route due to junction safety concerns.
“We apologise for any disruption caused and we are making every effort to complete the works as soon as possible so that the road can be re-opened to traffic.”
NYCC said those local to the bridge were issued with letters in August and September 2013 and in March 2014, though the mill centre owners said they were first notified only a week before the closure.
Signs indicating that businesses are open as usual were erected on April 4.
The owners of Darley Mill Centre said they worry that their recent investment in promotion was wasted, and that their charity duck race, held on April 27, will fail to raise its usual amount for deserving causes.
They are assuring customers they are still open, and have posted details of the shorter detour on their website.