The Scottish diplomat who wrote Article 50, the trigger mechanism for Brexit, says there will be a second EU referendum.

In an exclusive interview with the Herald on Sunday, John Kerr – who sits in the Lords as Baron Kerrr of Kinlochard – said: "The muddle we’re in now is one which can’t be sorted by March 29 [the date for Brexit] … I think we’re heading for a pause for thought, an extension of Article 50, and a referendum."

Kerr, who became involved in the independence referendum over the issue of Europe and campaigned for Better Together, says Brexit has nullified one of the key arguments for Scotland staying in the UK.

During 2014, Kerr argued that the Yes campaign was wrong to say Scotland could ‘just pull up another chair at the table’ with the EU if it left the UK. Kerr said at the time that an independent Scotland would have to apply to join as a new member meaning it would not get the UK rebate on contributions, worth £5.6 billion in 2017.

If the UK leaves the EU, however, ‘this argument is irrelevant’, Kerr says. However, he still believes financially Scotland is better off staying in the UK.

‘Being members of the UK multiples Scottish influence,’ he said. ‘If we Scots left the UK, the economic disruption would be even greater than the economic disruption if as members of the UK we leave the EU … Economists say the effect of leaving the single market of the UK, in terms of the damage to the Scottish economy, would be five times as great as the damage to the UK of leaving the EU.’

Kerr is also worried about the effect of English nationalism. ‘It’s the case that in Scotland, Wales and Northern Ireland, that to be dragged out because of an English nationalist vote against the EU would breed resentment unless there was another fair test of what the public as a whole wants.’

Kerr was scathing of how Britain carried out Brexit negotiations, saying it was not a ‘good idea’ to put Boris Johnson, David Davis and Liam Fox ‘in charge’. He believes Foreign Office officials should have played a central role but were cut out.

He said Brexiteer ministers ‘didn’t understand that [the 27 EU member states] would stick together, would stick up for Ireland, would defend the integrity of the single market. These were basic mistakes’.

The 76-year-old from Glasgow also criticised Theresa May, saying: ‘She didn’t reach out to Parliament, she’s never tried talking to the opposition to find a common position, she’s never reached out to the country and explained we can’t have our cake and eat it, and she’s never reached out to Edinburgh, Cardiff and Belfast and tried to form a consensus of the four nations.’

Renegotiation on any deal is unlikely, he believes. ‘I think the EU isn’t going to make many changes to the deal,’ he said.

On the issue of peace in Northern Ireland, Kerr said: ‘We’re in a very dangerous country when we’ve stories like the government thinking of going for some amendment to the Good Friday Agreement as a way of getting around the need for the backstop.’

He questioned whether ‘politicians in London quite understand how fragile the situation is’, and said the confidence and supply arrangement with the DUP made it difficult for a Prime Minister to appear an ‘honest broker, if you’re relying for your political survival on the support of one party which is very sectarian in its support’.

Fear of social unrest should not prevent a second EU referendum, he added. ‘The worst reason for not having a second referendum would be if we were frightened of right-wing populist gangs in the street,’ said Kerr, who believes Conservative austerity policies helped fuel Brexit in ‘left behind’ areas.

The UK will suffer on the global stage after Brexit, Kerr added. ‘If we go ahead and leave the EU I think we’ll pay a price that won’t just be economic - and it’ll be a serious economic price, particularly for poorer people in our society - it would also be political. I think our influence in the world as a whole would shrink.’