A group of Republican delegates in the US are reportedly attempting to block Donald Trump from securing the party's nomination for the US presidency.

The businessman is the party's presumptive candidate having secured a majority of delegates during the primary campaign. He is also the last active candidate.

However, the Washington Post reports that a coalition of GOP representatives is coming together to try and stop his nomination at the party's conference next month.

Quoted in the Post, Republican delegate Kendal Unruh said: “This literally is an ‘Anybody but Trump’ movement. Nobody has any idea who is going to step in and be the nominee, but we’re not worried about that. We’re just doing that job to make sure that he’s not the face of our party."

The delegates involved are reportedly pushing for a 'conscience clause' - which would mean they would no longer be bound to vote for Trump at the conference.

It comes as the Republican House of Representatives Speaker Paul Ryan told NBC News that he would not tell anybody "to do something that's contrary to their conscience".

"I get that this a very strange situation. [Donald Trump] a very unique nominee. But I feel as a responsibility institutionally as the speaker of the House that I should not be leading some chasm in the middle of our party," he explained.

Prominent Republicans have criticised Trump's comments on the Orlando shooting and the heritage of a Mexican judge, among other issues such as gun control.

Michael Graham has said: "Trump has waged open warfare on both conservative ideas (free trade, economic deregulation, free market health care) and conservative sensibilities."

The Republican party conference runs from July 18th until the 21st.