Gardening trade bodies are increasing their lobbying for an exemption for garden centres from Sunday Trading Act restrictions and the ban on Easter Sunday (5 April) trading ahead of the general election (7 May).
The rise of internet and TV shopping has left the 1994 law "not fit for purpose", they say. Garden Centre Association chairman Will Armitage, who represents 200 UK garden centres, said: "We are missing out on the opportunity and online shopping is certainly an inconsistency.
"You can go online and buy as many products as you like and some courier companies will even deliver them to your house on Easter Sunday, but you can't go to your local garden centre and buy anything. You can even click and collect from your local depot, but not the garden centre.
"All we call for is a level playing field with everyone either open or closed. That can't happen at the moment because of the rules and regulations. It depends on the size of your garden centre. Some local authorities allow you to open your restaurant in the centre and some won't. There's inconsistency in the way the law is interpreted in different regions.
"It's not about one garden centre competing with another but about other leisure activities being allowed to open on Sundays while garden centres can't."
HTA horticulture head Raoul Curtis-Machin said: "Sunday trading progress has slowed a little after a somewhat lacklustre debate in the House of Lords on a possible exemption for garden centres from the Sunday Trading Act.
"Many of the arguments that were used 20 years ago were put forward and there was very little recognition of changes in British society and working patterns.
Neither was there any recognition of modern influences on the retail world such as the internet - the debate didn't even mention the word 'internet', let alone the concept of omni-channel retailing and allowing people a choice. We shall continue with our efforts, however."
He added: "We have sent a document out to policymakers to show how garden centres could easily be exempted from the regulation and explaining how it places an unfair disadvantage on them in this day and age. We are waiting for a meeting date with a senior civil servant in the Department for Business, Innovation & Skills.
"We had one meeting with other civil servants in the same department and were told that there did not appear to be a political appetite for change. This is hugely frustrating because clearly there is a public appetite for change, as we will see yet again when garden centres have to employ staff on Easter Sunday to turn potential customers away.
"Shops such as RHS Wisley and the National Trust's garden centre at Morden Hall Park will have to keep the tills turned off, depriving charities of the potential to earn money."
Industry groups have employed lobbying companies to influence MPs to try and change the law. But some object for religious reasons, while others want to protect workers' rights.
Chancellor George Osborne and secretary of state for communities and local government Eric Pickles are believed to be sympathetic to a permanent extension of opening hours. But opposition from the Liberal Democrats and some Tory backbenchers is likely to scupper any change.
Legislation - Sunday Trading Act 1994
Shops in England and Wales with more than 280sq m of covered space cannot open on Sundays for more than six consecutive hours between 10am and 6pm and must close on Easter Sunday and Christmas Day.
Exemptions are for airport, rail station and service station outlets, registered pharmacies, farms selling mainly their own produce, outlets wholly or mainly selling motor or bicycle supplies, suppliers of goods to aircraft or sea-going vessels and exhibition stands selling goods.
- Around £75m of garden centre sales are lost to the economy a year because of Sunday trading regulations.
- On Easter Sunday, when garden centres are unable to trade at all, an average store loses £5,525.
- Given that there are around 2,200 garden centre outlets in England and Wales, this represents £12m in lost sales on one day.