'I actually think a lot of Nos are going to go in there, and say 'it's too risky, too risky', and then just go 'f*** it' and go Yes'

"Going to the booth is going to be like losing your virginity," he says. "You are a bit nervous and a bit scared. But I think once you see somebody with their clothes off, you think, 'I'm just going to get f****** fired in there'."

He adds a caveat: "It will also be over very quickly."

Following his logic, might some people not bottle it?

"I think it will be 'BANG'," he counters. "It won't be the greatest experience they've had, but they'll manage to get the cross down."

Welsh, 56, is among a range of Scottish writers and artists to back a Yes vote next month.

The Leith-born writer shot to fame in the 1990s with his iconic debut novel Trainspotting - a wonderfully scuzzy account of drug addiction and the Edinburgh schemes - and he has since published eight more novels, including Porno, a sequel to Trainspotting.

Welsh won't be voting - Chicago and Miami are his current homes - but he was back in the capital earlier this month to see friends, do an event at the Edinburgh Book Festival and lap up the referendum atmosphere.

Regardless of the result next month, he believes independence will happen.

"I think it's going to continue," he says, in a café in the New Town. "There's a whole new generation who have been politicised by the Yes campaign. You have to have something to hold an entity together. I don't think there's anything holding the UK together now."

He is unsure of how Scotland will vote, but how about his native Edinburgh?

"It's a strange place," he says. "There are so many different elements. It just depends on who comes out to vote on the day. If you can get the working class out and get people coming round the schemes, I think there will be a really big Yes."

In some ways, Welsh is a typical Yes supporter: he is on the Left, disillusioned with Labour, and sees Westminster as having no, or few, redeeming features.

Equally, his support for independence is complex. He has written of a "strong emotional connection" to Britain as a result of spending time in the 1970s with relatives in Southall, Middlesex. However, his sense of British identity is invested in institutions he feels have been undermined.

"Labour built the welfare state and that was one of social high points of Britain," he says. "Since then, the whole idea of Britain has been in decline. We've had de-industrialisation, the decline of the empire, the decline of the welfare state, the breaking up of the NHS and the movement of the Labour Party from being a party of the Left … It has been a series of changes."

He also feels the "punk generation" - some of whom reaped the spoils of Thatcherism - owes independence to today's young people.

"I don't think we've got any right to say to people, 'You can't have this', because we have left them with f*** all," he says. "Thirty-five years of neo-liberalism happened under our watch … All these things, we kind of benefited from in some ways. It's been lost to the next generation. I think we have got a responsibility."

Welsh makes a distinction between the neoliberal "UK state" and social democratic Scotland, but I point out the SNP is bankrolled by Brian Souter, who capitalised on the privatisation of the buses and who has reactionary social views.

Isn't it more complex than Welsh makes out? "It is very, very complex," he says. "There are a lot of crusty old reactionary elements in Scottish society. Some of them in the No side, some of them are on the Yes side."

The London Olympics opening ceremony, masterminded by Trainspotting director Danny Boyle, focused heavily on UK achievements like the NHS. Couldn't Boyle's work be seen as a symbol of progressive Britain?

"It was fantastic, it was great," Welsh says of the ceremony. "A lot of people I know, English pals who are on the Left, had the same kind of thing that I did. They felt really angry and really sad about it.

"It was almost like a Requiem Mass in some way for something that was lost, as well as a celebration of the good things in Britain.

"I talked to Danny [Boyle] about it afterwards, and he was saying it felt a bit like that as well. You were trying to dredge up the good things that have happened in Britain, but in doing that you were realising how much had been taken away."

Ultimately, Welsh says, Scotland and England are on different political paths: "It just seems to me that the political cultures have changed so much in the two countries. England is on a different mission. It is a populist multi-ethnic country, it's trying to make itself into a post-imperial democracy, and it does have issues about immigration and its borders. We have issues the other way with immigration - we are a constantly de-populating country."

One unproven claim about independence is that, somehow, it might be better for writers and artists. Is he persuaded? "It's difficult to say. You wonder what is the best regime for artists and writers. Is it a regime you actually hate, or is it a regime that's quite sympathetic to you?

"After the referendum vote in '79, you had a big explosion of writers and artists. The frustration kicked in about not having that settlement recognised. The ideal position for me would be to have a Scottish Government that I could be massively critical of. That it [the government] just be a total pain in the arse."

Welsh's fellow novelist, Alasdair Gray, was criticised when he divided some English people living in Scotland into "settlers" and "colonists". What did Welsh make of the debate?

"People get a bit too fetishised about English people living in Scotland and Scottish people living in England. There are bigger political issues," he says.

"I think his point was that you have a small nation that in some ways is internally colonised, and if you have people who are not empathetic towards Scottish culture and the Scottishness of things you dilute that and you bland out your own culture."

Perhaps Welsh's greatest achievement as a novelist is his creation of memorable fictional characters.

If Begbie, Renton and Spud from Trainspotting had a vote next month, which way would they go?

"Spud would probably be Yes, but get lost on his way to the polling booth," he laughs. "Begbie is difficult to say. I think he would probably tell everybody he wasn't going to vote and it was a load of sh***, and then probably vote Yes. I think Renton would be Yes."

How about Juice Terry, the work-shy "fanny-merchant" in his novel Glue? "Yes. Because he says yes to everything."

And Bruce Robertson, the bigoted cop in Filth? "Bruce would be No."

Would Welsh move back to Scotland in the event of a Yes vote?

"These things are not really related," he says. "I would like to. I've lived in England, in Holland, in Ireland, in Midwest America and the west coast of America. I never really planned any of that to happen. It's probably always going to be like that for me."

On balance, I sense Welsh would not bet his mortgage on a Yes victory next month. However, he believes the "f*** it" factor could be the wild card on the day: "I actually think a lot of Nos are going to go in there, and say 'it's too risky, too risky', and then just go 'f*** it' and go Yes."

In 18 days, Scotland will find out whether Welsh's unconventional theory of electoral behaviour will deliver a geo-political tremor.