European officials have poured cold water on hopes that Theresa May could negotiate Brexit with other EU leaders in September to break the deadlock over Britain’s departure.

Diplomatic sources have rejected suggestions that May could hold direct talks on Brexit with the 27 other EU heads of state and government at a summit in Salzburg next month.

“That is completely ridiculous, that is complete overspin of Salzburg,” one senior source told the Guardian. “It would mean that we would ditch our negotiating approach of the last two years and discuss at 28 instead of 27 to one, and I don’t see why this would happen.”

Brexit talks are due to resume in Brussels on Thursday and Friday, the start of a new intense phase of negotiations, with the aim of reaching a deal in the autumn.

Since the referendum, the EU has insisted that all formal talks are led by the chief negotiator, Michel Barnier. May is allowed to update EU leaders on her plans at quarterly EU summits but is not in the room for discussions.

Officials expect this approach to be continued at Salzburg, an informal summit on 20 September officially dedicated to migration. The meeting has been organised by Austria, which currently holds the EU rotating presidency, but it will be for the European council president, Donald Tusk, to decide whether to add Brexit to the agenda.

The Salzburg gathering comes four weeks before an EU summit in Brussels, pencilled in by Barnier as the moment to strike a deal. Many in Brussels expect the deadline to slip to November or even December, squeezing the time available to ratify the text ahead of the UK’s departure on 29 March 2019.

The British government has tried to get round Barnier by appealing directly to national capitals for greater flexibility, a strategy continued by the foreign secretary, Jeremy Hunt, who is on a three-day Brexit tour on the continent this week.

Latvia’s foreign minister, Edgars Rinkēvičs, said before meeting Hunt that there was a 50-50 chance of failing to reach a deal. “I believe both the EU and the UK need to have extra effort to reach some kind of deal,” he told BBC Radio 4’s Today programme.

Barnier’s mandate is set by the EU’s 27 leaders and there is little appetite to change it, despite Hunt’s calls for a new approach.

No one is expecting a dramatic breakthrough from this week’s low-key Brexit negotiations in Brussels, which will be led by officials rather than Barnier and the Brexit secretary, Dominic Raab.

Last month Raab said that from mid-August there would be “weekly discussions to clear away all the obstacles that lie in our path to a strong deal in October”.

On Thursday negotiators will discuss the problem of the Irish border, with the two sides deeply divided over how to avoid customs checks at the crossing. The second day will be devoted to discussing the UK’s future relationship with the EU, outlined in May’s Chequers blueprint.

The EU sees serious problems with the Chequers plan and has dismissed the customs ideas as unworkable. However, EU negotiators have not entirely written off May’s plan and see the kernel of a free-trade agreement. But this outcome would fall far short of the prime minister’s hopes for deeper economic ties.

EU officials insist they cannot break up the EU’s single market and its “four freedoms” of goods, capital, services and people. “I can’t imagine we would walk away from that,” said one diplomat. “It is the raison d’être of the EU. We can’t have Brexit threaten our political and economic system.”