Senior Tories have urged Theresa May to set out a clearer plan for Brexit, as yet another EU leader warned Britain was not going to get special deal on immigration and the single market.

Nicky Morgan, the former education secretary, and Ken Clarke, the former chancellor, pressed May to come up with more than her slogan “Brexit means Brexit”, ahead of her first party conference speech as leader in Birmingham next week.

Meanwhile, the Italian prime minister, Matteo Renzi, said there would be an “interesting debate” about the single market and immigration across Europe but added: “It will be impossible to give to British people more rights than other people outside the EU.”

Renzi also criticised David Cameron for calling a referendum to sort out internal party problems.

May has stuck to the line that she will not be giving a running commentary on her negotiating position, although she has indicated she would like a bespoke deal allowing curbs on free movement at the same time as access to the single market.

Morgan, who is emerging as a leading Tory moderniser on the backbenches, told BBC Radio 4’s Today programme that the prime minister should provide more details about when she would trigger article 50 to start the process of leaving the EU at the party conference next week or certainly within the next couple of months.

“There has been a lack of a plan coming from the government and, yes there is a danger where the government will find itself in a position where other people are setting the terms,” she said.

Morgan was backed up by the former Tory business minister and remain campaigner Anna Soubry, who said the former education secretary was “right that we need to know the principles behind the government’s Brexit plan”.

Ken Clarke, a longstanding pro-EU Tory, also weighed in, claiming May was running a government without any policy on Brexit.

“Nobody in the government has the first idea of what they’re going to do next on the Brexit front,” he said in an interview with the New Statesman.

In a separate development, a German business group, the Federation of German Industries, said it would be better for the UK to pursue a so-called hard Brexit that involves fully leaving the single market.