DAVID Davis has been branded “thick as mince” by a former head of the Vote Leave campaign as the Brexit Secretary also came under fire for leaving the high-profile EU talks in Brussels after just three hours.

The outburst from Dominic Cummings, a former special advisor to Michael Gove, the Environment Secretary, reflected the continuing rancour between different wings of the Brexit movement, which saw intense infighting during the EU referendum campaign.

In a stream of messages on Twitter, he warned that a little-noticed clause in the EU [Withdrawal] Bill could allow UK Government ministers to alter their Brexit position at the last minute to secure a lengthy transition period.

It is thought Mr Davis and Chancellor Philip Hammond favour a lengthy transitional period of around a “couple of years” while the likes of Mr Gove and Liam Fox, the Trade Secretary, prefer a much shorter one, lasting just a few months.

In a deeply personal attack, Mr Cummings, who was the campaign director of Vote Leave, tweeted that the Brexit Secretary was "thick as mince, lazy as a toad and vain as Narcissus".

He claimed Mr Davis "spent the campaign boozing with[Nigel] Farage, predicting defeat and briefing [against] Vote Leave" and was involved in the "single c***pest TV news for Leave of entire campaign" when Grassroots Out unveiled George Galloway, the former MP, as a supporter.

Mr Cummings insisted his latest comments were not prompted by any falling out between Mr Gove and Mr Davis, insisting he had not spoken to the Environment Secretary about his Cabinet colleague since before the referendum.

Earlier, the Brexit Secretary was in Brussels for the start of the second round of talks with his EU counterpart Michel Barnier, saying how they were now “getting into the substance of the matter”. They are due to focus on the “divorce bill,” citizens’ rights and Northern Ireland.

But after just three hours, Mr Davis was heading back to London. It was also noticed how he and his team were photographed sitting around the negotiating table without any papers while facing them were Mr Barnier and his team with stacks of documents.

Sir Keir Starmer, the Shadow Brexit Secretary, said: “David Davis can hardly say this is the time ‘to get down to business’ and then spend only a few minutes in Brussels before heading back to Whitehall.

"We need a fresh approach and to see real progress in negotiations. That means engaging with the substance of talks and resolving vital issues such as citizens’ rights, that have already dragged on for too long.”

Tom Brake for the Liberal Democrats accused Mr Davis of having “skulked back to the UK after just half a day”.

He added: "He didn't have any position papers with him because this Government has no agreed Brexit position; this is a Government with no papers, no plan and no time for the most important negotiations of a lifetime.”

Mr Davis is due to return to the Belgian capital for more talks on Thursday.

Meanwhile, the Scottish Government issued a paper, responding to the UK Government’s proposals on the status of EU citizens in the UK.

Prime Minister Theresa May has insisted no EU national currently resident in the UK will have to leave at the point of Brexit and that if they have been living here legally and continuously for at least five years they will be offered so-called “settled status”.

Among the questions the Scottish Government asks is: what does “settled status” precisely entail; how much will it cost; what will be the rules for family members of EU citizens and what will be the cut-off date for those on a path to settled status.

Michael Russell, the Scottish Government’s Brexit minister, said: “EU citizens make a vital contribution to Scotland and to our economy, society and culture. They must have clarity about their future rights and what Brexit will mean for them and their families.”

At Westminster during a Lords debate, Lord Green, chairman of Migration Watch UK, the think-tank which campaigns for tighter immigration controls, told peers net migration to the UK could be cut by about 100,000 a year post-Brexit through a work permit scheme for EU workers.