The Notting Hill Carnival is held over the August bank holiday weekend
Minister Liam Byrne has been accused by the SNP of a "glaring gaffe" over his choice of date for a "British day".
Mr Byrne was due to call for the "August bank holiday" to be made a weekend of national celebration in a speech to a New Labour think tank.
But he back-pedalled after the SNP pointed out that the August holiday is on a different date in Scotland to what it is in England and Wales.
Mr Byrne said he had just been trying to "get the debate started".
In the text of the immigration minister's speech to the Progress think tank, released in advance to the media, Mr Byrne said there was public support for a British day but disagreement over when it should be held.
"I myself have become convinced that the August Bank Holiday weekend - what some have called 'the great British weekend' - has the virtue of being in the summer, and already being a bank holiday," the text said.
The speech and accompanying press release were seized on as a "blunder" by the SNP, with the party accusing Mr Byrne's plans of falling "at the first hurdle".
The August bank holiday is at the start of the month in Scotland and at the end of it in England and Wales, the party pointed out.
Labour's cack-handed attempts to resuscitate a British 'national' identity are just desperate, motivated by self interest rather than national interest
"That glaring gaffe aside, Liam Byrne's proposal has nothing to do with Britishness, and everything to do with bolstering Gordon Brown as Labour's support evaporates in swing English regions," said Angus Robertson, leader of the SNP at Westminster.
"Labour's cack-handed attempts to resuscitate a British 'national' identity are just desperate, motivated by self interest rather than national interest."
But Mr Byrne defended his call for a national day "towards the end of August" in an interview with BBC Radio 4's PM programme.
'Really caught on'
He denied excluding Scotland from the proposed celebrations, saying: "All I am trying to do in this speech that I am making a little bit later on is actually get this debate started.
"I don't think there is any real consensus in Britain at the moment about when the right day is but actually I think it's a good idea - because actually I think people are looking for an opportunity, permission if you like, just to celebrate the things that they like about living in Britain.
"I think it should be in the summer towards the end of August."
He stopped short of suggesting the English and Welsh bank holiday should be extended to cover Scotland as well.
"There is a different question about whether the country needs another day off. All I'm saying is that I think a day towards the end of August might actually be a good idea.
"It could well be that there is an argument for extending a national bank holiday, that is something the government is still thinking about."
Mr Byrne said the idea of a national British day had "really caught on" in consultations he had held with voters.
In his speech, he will argue that it should combine national and local events to give people the opportunity to celebrate "the best of being British" and "showcase all the colours of the British tapestry".
But an aide suggested he may change the text to say that the final Monday in August was the best choice as it was already a bank holiday "in some parts of the UK".
Mr Byrne's proposals have also come under fire from the trade unions, who are campaigning for a new bank holiday in October.
The TUC said a day of national celebration was a "great idea" - but a spokesman said it risked being rejected if it was held on an existing public holiday.
"It is a great idea to have a day celebrating Britishness, but it would be a shame that millions of people will potentially miss out as they will already have made plans to take part in other events.
"We think it would be a lot more effective if they introduced a new bank holiday.
"People already have an idea what the August Bank Holiday means - it is the Notting Hill Carnival, or the music festivals."
He called on the government to be "a bit more ambitious and bring in a new bank holiday that everyone can enjoy".
The TUC is campaigning for a new bank holiday in late October, to break the run of 16 weeks without a holiday between August and Christmas.
The spokesman said Britain had the second lowest number of public holidays in Europe - and he rejected claims by the Confederation of British Industry that a new holiday would cost £6bn in lost production.
As an advanced industrial nation, Britain could "easily afford it" and it would generate income for many firms, he argued.
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