Donald Trump has won enough delegates to secure the Republican party's nomination for US President, according to Associated Press.

The controversial billionaire has now reached 1,238 delegates, one more than is needed, AP reports. He reached the quota after a handful of the party's unbound delegates told AP they would pledge their support.

Trump's bombastic campaign has riled the Republican party's elite and he has struggled to win support from those at the top of the party. Many senior party members had hoped he would not win enough delegates to clinch the nomination outright which would force a second round of voting at the party's national convention in July.

Towards the later stages of his campaign, it was clear rivals Ted Cruz and John Kasich could not win enough delegates to seize the nomination but they hoped by remaining in the race they could prevent Trump from winning outright and claim the nomination at the convention.

That was put to bed after Trump won in Indiana, taking all the state's 57 delegates, and both Cruz and Kasich withdrew their bid.

The majority of Republican delegates are bound in who to support by their state's voting results. However a few delegates remain unbound going in to the convention and it is these unbound delegates that have carried Trump over the line, according to AP.

One of these is Oklahoma GOP chairman Pam Pollard. "I think [Trump] has touched a part of our electorate that doesn't like where our country is," she said.

"I have no problem supporting Mr Trump."

The New York billionaire has amassed millions of fans among grassroots Republicans despite the leadership's reluctance.

Another unbound delegate, the chairman of the Colorado Republican party Steve House, said he would support Trump because of his business background.

"Leadership is leadership," said House. "If he can surround himself with the political talent, I think he will be fine."

Trump is expected to be officially announced as the Republican candidate at the July convention after one round of votes.