DOWNING Street has warned Nicola Sturgeon not to do anything that would “undermine” the UK Government’s negotiating position on Brexit.

As Theresa May chaired the two-hour meeting of the Joint Ministerial Committee on how Britain will leave the European Union, her spokeswoman stressed how the UK Government would fully engage with the devolved administrations, offering the possibility of a “direct line” to Whitehall with a new intergovernmental forum, but that there had to be a unified approach and a “single UK position”.

Asked about the possibility of a “flexible Brexit,” where different parts of the UK had different terms as the country left the EU, she said: “We are clear a single UK position is vital to protect the UK’s interests as a whole and we have to make sure we are not putting up barriers to trade within the UK.”

Read more: Theresa May warned imposing Brexit settlement on Scotland could break up Britain

When it was pointed out that intergovernmental relations over Brexit had not got off to a good start given UK Government ministers had already ruled out those things the First Minister had said the Scottish Government wanted - including Scotland to remain in the single market, a Holyrood vote on the UK strategy and more powers for the Scottish Parliament over immigration and trade – the spokeswoman said: “We want to engage, we want to listen to proposals put forward while being clear the approach taken must advance the UK as a whole.

“It does mean approaching this as one United Kingdom. On issues like immigration, the Smith Commission looked at the new powers Scotland should have as part of its devolution settlement and did not recommend devolving immigration.”

The Herald reported last week that SNP sources had suggested that, if any of the Scottish Government’s proposals were incorporated into the UK Government’s negotiating position, then Ms Sturgeon would want to be at the talks table in Brussels.

When asked if there was a chance that any representatives of the devolved administrations could be part of the UK negotiating team, the PM’s spokeswoman said: “We’re having the first JMC in two years today. The Prime Minister’s first visit outside of London was to go and meet with the First Minister of Scotland to talk about these issues and subsequently with other first ministers and this forum will provide the opportunity to further engage in the discussions and negotiations.”

In light of Ms Sturgeon’s recent announcement of creating more Scottish trade envoys, Mrs May’s spokeswoman was asked if it would be helpful for there to be parallel conversations between Scottish Government representatives and the EU. She replied: “We have been very clear about that we should be working together to secure the best possible deal for the whole country and we expect representatives of the devolved administration to act in that way and in no way to undermine the UK’s position.”

Read more: Theresa May warned imposing Brexit settlement on Scotland could break up Britain

Asked, should the PM’s talks with Ms Sturgeon on a UK approach fail, how seriously Mrs May was taking the threat of a second Scottish independence referendum, her spokeswoman replied: “The PM is focused on getting the best deal for the whole of the UK and approaching these negotiations in that way.”

The devolved administrations are keen to secure continued participation in the single market and want to hold votes on the PM’s approach before she triggers Article 50, formally beginning the Brexit process. However, UK ministers have made clear this is a non-starter as any vote could veto Whitehall’s preferred approach.

The PM has offered them a "direct line" to Brexit Secretary David Davis, who, it is proposed, will chair a new forum bringing together representatives from Westminster, Holyrood, Cardiff and Stormont for regular talks on the situation.

Ahead of today’s talks, Mrs May said: "I am determined that as we make a success of our exit from the European Union, we in turn further strengthen our own enduring union.

"The great union between us has been the cornerstone of our prosperity in the past - and it is absolutely vital to our success in the future.

"The country is facing a negotiation of tremendous importance and it is imperative that the devolved administrations play their part in making it work.

"The new forum I am offering will be the chance for them all to put forward their proposals on how to seize the opportunities presented by Brexit and deliver the democratic decision expressed by the people of the UK," she added.

But leaders in Wales and Scotland have called for the UK Parliament and the three devolved legislatures to be given their own votes on the negotiating position the Government intends to take and said that Article 50 should not be triggered until there is an agreed approach.

Read more: Theresa May warned imposing Brexit settlement on Scotland could break up Britain

"We believe that a UK Negotiating Framework should be developed, based on principles and aims [but without revealing a detailed 'negotiating hand'] and this, linked to invoking Article 50, should be the subject of a vote in all four of the United Kingdom's parliaments and assemblies," Mr Jones said in a letter to the PM.

"Such an approach would properly reflect the stated position of the UK Government that the UK is a family of nations, a partnership of equals."

He also said the final "exit deal" should also be subject to votes in all four UK parliaments and assemblies.

Ms Sturgeon backed Mr Jones' position, telling the Prime Minister: "It will not be acceptable for the devolved administrations to simply be consulted on UK Government plans. We must have meaningful input into the decision making structure and the formation of negotiating positions."

Meantime, the think-tank, the Institute for Government, warned Mrs May about seeking to impose a Brexit settlement on the devolved nations, saying this would be a “reckless” strategy. However, it also said that the devolved administrations had to accept that Westminster would have the final say.