Jeremy Hunt has been accused of trying to boost his Conservative leadership campaign by withdrawing Foreign Office (FCO) support for Nicola Sturgeon’s overseas visits.

The foreign secretary has confirmed that he has ordered his department not to provide any official or consular support for Sturgeon’s trips abroad if she uses them to campaign for Scottish independence.

The row first erupted when the FCO refused to support Sturgeon’s trip to Brussels to meet Jean-Claude Juncker, the European commission president, and Michel Barnier, the EU’s chief Brexit negotiator, two weeks ago. The first minister gave a speech attacking Brexit and extolling her government’s pro-EU, pro-independence stance.

Hunt has told the Scottish Daily Mail during a leadership campaign visit to Scotland that the policy would affect Sturgeon’s future tours and visits, although he indicated it would apply primarily if Scotland’s constitutional future was on the agenda.

“I believe in the union with every fibre of my being and British government policy is to protect, preserve, cherish and enhance that union,” Hunt said. “So we are not going to offer support to Nicola Sturgeon if she goes abroad to drum up support for independence.”

Sturgeon’s official spokesman said it was extraordinary for a candidate for prime minister to withdraw support offered to every devolved administration and minister, while at the same time claiming to be protecting the UK as whole.

Hunt’s stance was “the most visible way possible of making the arguments for independence”.

“It is entirely legitimate for the democratically elected first minister to espouse the views that she was democratically elected on,” the spokesman said. “She is not going to take a vow of silence when [the media] ask her about independence.”

He added: “It would be naive to think this was nothing to do with the Tory leadership race.”

Sturgeon’s trip to Brussels was not directly affected because the Scottish government has its own external affairs office in the Belgian capital, he said; it would be considered much more serious if the policy affected future trips to other countries.

She has been on FCO-supported trade visits to China, the US, Canada and other countries, frequently collaborating with Scottish universities, as well as technology and whisky companies and other business bodies.

However, on trips to Brussels, Dublin and North America she has given speeches and media interviews that focused heavily on her government’s objections to Brexit and her quest for independence, including plans to potentially rejoin the EU in future.

“Let’s wait and see exactly how official or otherwise this is, or if this is a flash-in-the-pan leadership pitch from Jeremy Hunt which fizzles out. The jury is out on that one,” her spokesman said.

“Longer term, of course we would be concerned if the Scottish government was not to be afforded the same level of representation as the rest of the UK.”