It is “grossly unfair” that supermarkets can sell greeting cards in lockdown while specialist retailers have to shut their shops, the boss of Clintons cards has told the BBC.
Eddie Shepherd said some retailers were “exploiting” ambiguities in the rules.
Until 2 December, any shop in England selling “non-essential” goods such as gifts, books and homeware must close.
But those deemed essential can continue to sell non-essential items if they are stocked on their aisles.
It has sparked a wave of complaints against supermarkets, garden centres and news agents, with the boss of book chain Waterstones saying the government guidance “doesn't make sense”.
Mr Shepherd told BBC Radio 5 Live's Wake up To Money programme: “If our category and product is deemed to be non-essential, which it is, then that should apply in all retail scenarios.
“But garden centres and supermarkets often operate card and gift sections as large as some High Street stores and they are able to continue to trade in these sections whilst we're not.”
He added: “I think elements of the legislation are unclear and it's affording an ambiguity that people are able to exploit.”
Clintons' sales dropped sharply in the first national lockdown and the chain – which has 270 UK shops – hoped to make up for it this Christmas.
But Mr Shepherd said revenue would now be 20% of usual levels this December, in part due to trade lost to essential retailers.
“Undoubtedly an element of what was the available market will be gone at the point we reopen,” he said.
Clothes and book sellers have also criticised the lockdown rules in England, with the British Retail Consortium (BRC) accusing the government of creating “arbitrary” lines between retailers.
During Wales's recent two-week lockdown, essential retailers had to cordon off aisles selling clothes and toys, although this sparked anger among some customers.
‘Hurts the independents'
James Daunt, the boss of book chain Waterstones, has repeatedly criticised the fact that WH Smith continues to sell books in its shops in lockdown because it is a news agent, while his business can only sell online.
On Monday, he told the BBC: “I don't think anyone would object to the supermarkets being open to sell food and pharmacies to sell medicines.
“What I am objecting to is really very comparable retailers are open, and others closed, and I think that really hurts the independents.”
The BRC estimates non-essential retailers in England will lose £2bn of sales in the lockdown, which began on 5 November.
However, the Department For Business, Energy and Industrial Strategy maintains the new restrictions will limit social contact and slow the spread of coronavirus.
“We recognise this continues to be a very difficult period for businesses, which is why we've confirmed that there will be a full package of financial support in place,” a government spokesperson said last week.