Northern Ireland could lose out on a Sunday tourism spend of around £3m every week as a result of not relaxing trading restrictions.
The warning was issued 10 days ahead of the opening of Titanic Belfast, when thousands of visitors are expected to descend on the city.
Retailers are currently only allowed to trade between 1pm and 6pm on the Sabbath — and Stormont has said there will be no concessions during the centenary commemorations of the 1912 maritime tragedy.
Experts have expressed fears that the decision not to follow the UK Government — which intends to relax restrictions during the London Olympics — could mean the loss of millions of pounds to the local economy at a time when it can least afford it.
University of Ulster-based retail commentator Donald McFetridge has urged the Executive to reconsider its position on Sunday trading times.
“Northern Ireland's economy is really struggling to get back on its feet,” Mr McFetridge said.
“With the Titanic celebrations in April expected to kick off an extremely important year in tourism here, it is important to be as open for business as possible.
“If retailers had the option of opening up for longer on Sundays this would encourage visitors to spend millions of pounds more,” he said.
“It would also allow traders to increase their turnover potential, as well as provide employment opportunities to those who are crying out for work.”
Titanic Belfast’s marketing executive Claire Bradshaw has dubbed the new £97m Titanic building as “our Guggenheim” and a “world-class tourist attraction”.
And, with its nine galleries spread over four floors and an indoor amusement ride, the striking riverside development is at the centre of an unprecedented marketing push.
An advertising campaign, NI 2012: Our Time, Our Place, has been launched by the Northern Ireland Tourist Board in 22 countries to cash in on the unprecedented number of major events happening this year.
It features, among others, our Major golf winners Rory McIlroy, Darren Clarke and Graeme McDowell.
Titanic Belfast is said to be projecting 425,000 visitors this year, rising to 600,000 by 2017.