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The government faces possible defeat on plans that could extend Sunday trading hours in England and Wales.

MPs will vote later on handing control of the issue to local councils - which unions and Labour say could extend Sunday opening by up to six hours.

More than 20 Tory MPs are expected to rebel, one saying the shake-up would "chip away" at Sunday's special status.

Ministers have offered concessions, including piloting the changes and an annual review, if MPs agree to them.

If the SNP bands together with Labour and the 20 or more Conservatives backbenchers opposed to the plans it could defeat the government in the Commons vote - expected at around 18:00 GMT.

Pilot idea

The SNP had considered abstaining in the vote, as there are no Sunday trading restrictions in Scotland, but now plans to vote against. It claims the plans could hit shop workers' pay in Scotland - a position described as "extraordinary" and "hypocritical" by the government.

In the hope of averting a possible defeat, the government has offered to trial the changes in 12 areas. It says their impact on communities and workers will then be evaluated after a year before MPs decide whether to extend them throughout England and Wales.

Media playback is unsupported on your device Media caption BBC News spoke to some shoppers on Oxford Street what they think about whether shops should open longer on Sundays

An amendment to the Enterprise Bill promising this was not selected for debate by the Commons Speaker, who told MPs that it had been submitted too late, but ministers have said they will table it in the House of Lords in the coming weeks.

Conservative MP for Enfield Southgate, David Burrowes, who is among those planning to vote against the changes, said he was sorry that it had come to this, given that the PM had ruled out changes to existing Sunday laws during the general election campaign.

He said Sunday trading hours were "an issue of conscience" and challenged the need for the shake-up, saying retailers believed "sorting out parking and business rates" was more important for regenerating High Streets.

Media playback is unsupported on your device Media caption Tory MP urges government to keep Sunday special

"Why are we trying to unpick something which is fairly settled?" he said. "There are complex arrangements about Sunday trading and it is a duty on us to look at it carefully, consult on it widely and to scrutinise it fairly. None of those things have happened."

For Labour, shadow business minister Bill Esterson said the government had not made the case for changing the law, and that allowing stores in some areas to open for longer would lead to a "domino effect" elsewhere.

And the SDLP's Mark Durkan said the government's "artist's impression of a flat pack compromise" should not be taken seriously.

'Trusting the public'

However, Conservative MP for Newark, Robert Jenrick, said the public should be trusted to vote with their feet.

"It isn't actually the most complicated decision constituents will make in their lives," he said. "Shouldn't we trust our constituents to make wise decisions for them and their families?"

Planning minister Brandon Lewis said it was "not forcing anyone to shop on a Sunday" but the existing laws were devised "before anyone had heard of Amazon" and he defended the government's right to modernise them.

"Our election manifesto was clear we wanted to see economic growth and the prime minister was very clear at this dispatch box last year that we feel it it is time to review Sunday trading laws in light of how things have moved on," he said.

The BBC's Nick Eardley on the situation in Scotland

Media playback is unsupported on your device Media caption Neil Gray explains why SNP MPs will vote against trading hours changes in England

There are no specific restrictions in Scotland on when big shops can open - though in some religious communities they don't open on a Sunday.

Historically, many staff have been able to negotiate higher rates of pay for working on Sunday. Not everyone is entitled to this - many younger workers don't get any extra - but the STUC estimates that for a third of retail workers, the premium is worth £1,400.

The SNP says it is concerned those premiums would be eroded if the whole of the UK moves to unrestricted Sunday trading.

But their opponents accuse them of hypocrisy, saying shoppers in England and Wales should be given the same freedom as those in Scotland.

The SNP said some Scottish workers were paid extra for working on Sundays and there were no guarantees these "premium pay" rates, worth thousands of pounds a year, would be protected if hours were extended in other parts of the UK.

"We are relying on guarantees or not from private businesses and that simply doesn't wash when we are talking about the incomes of perhaps a third of retail employees in Scotland," its deputy leader Stewart Hosie said.

'Toxic' issue

The Lib Dems have been given a free vote - with their eight MPs expected to be split 50/50 on the issue.

BBC deputy political editor James Landale said Sunday trading had long been a "toxic" issue for the Conservatives, causing a rare Commons defeat for Margaret Thatcher in 1986, when 72 Tory MPs rebelled against plans to relax restrictions. Read more from James.

Since 1994, small shops - those up to 280 sq m, or 3,000 sq ft in size - in England and Wales can open when they want to on Sundays but larger stores are restricted to six hours between 10:00 and 18:00. Retailers can be fined up to £50,000 if they break the rules.

Ministers want to give the 353 councils in England and 22 in Wales the freedom to determine opening hours in their area.

More than 100 Tory councillors voiced their support for the move urging the government to "help us deliver what is best for our local communities".

But critics argue that family time and other pursuits, including Church going and other forms of religious observance, should be prioritised.

A group of faith leaders, including the Bishop of St Albans, the Archbishop of Wales and the President of the Methodist Conference, have warned the changes will "disrupt the rhythms of community life that are so integral to the common good".