Former Presidents George H. W. Bush and George W. Bush will not endorse Donald Trump, their spokesmen have announced.

Presidents 41 and 43 said they would not back Trump, despite him becoming the presumptive nominee after his two Republican rivals dropped out of the race for the White House.

Both of the former commanders-in-chief joined a growing list of prominent Republicans who have refused to back Trump's run for the presidency.

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Former Presidents George H. W. Bush and George W. Bush (pictured in 2013) will not endorse Donald Trump

Former President George W. Bush, whose brother Jeb pulled out of the race in February after a string of dismal results, will not be taking part in the campaign whatsoever.

When asked if the most recent Republican president would throw his significant political weight behind Trump, his spokesman Freddy Ford said: 'President Bush does not plan to participate in or comment on the presidential campaign.'

Former President George H. W. Bush's spokesman Jim McGrath said: 'At age 91, President Bush is retired from politics.'

Donald Trump has all but wrapped up the Republican nomination after Texas senator Ted Cruz and Ohio governor John Kasich dropped out of the race following the Indiana primary on Tuesday.

However, some GOP members are still holding out for a contested convention, which could happen if Trump does not secure the required 1,237 delegates.

He currently has 1,053 delegates, but is likely to secure most of remaining 514 still available as he has no competition.

Trump is now the Republican presumptive nominee after his two remaining rivals Ted Cruz and John Kasich dropped out of the race for the White House

Not a popular choice? Several Republican big hitters have said they won't back him or have remained silent, including House Speaker Paul Ryan

Republican senator Ben Sasse, of Nebraska, reiterated statements that he would not back Trump and said he would look for an alternative candidate if Trump became the nominee.

There was notable silence from House Speaker Paul Ryan, who has spoken critically of Trump but has refused to run as an alternative candidate.

Tim Miller, a former spokesman for Jeb Bush, tweeted: 'Never ever ever Trump. Simple as that.'

Republican Senator Deb Fischer madeclear in an interview with Nebraska Radio Network that she wouldsupport the party's nominee but was not comfortable with Trump.

'Mr Trump is going to have to work hard to bring the partytogether,' she said.

South Carolina's Republican governor, Nikki Haley, issued a statement saying she would support the Republican nominee but was 'not interested' in being the party's vice presidential running mate after there were rumors Trump might call on her.

But Oklahoma Republican Governor Mary Fallin endorsed Trump enthusiastically and welcomed talk of her as his possible No. 2.

Senate Majority Leader Mitch McConnell said Trump has the 'opportunity and the obligation' to unite the GOP.

He said the party was committed to 'restoring economic and national security' and preventing a 'third term of Barack Obama' if Democratic front-runner Hillary Clinton wins the White House.

McConnell called on Trump to unite the party around 'our goals'.

Former President George W. Bush (pictured with his father and Jeb Bush in 2009), whose brother Jeb pulled out of the race in February after a string of dismal results, will not be taking part in the campaign whatsoever

It came as hundreds of Republican supporters set fire to their voter cards and declared support for the #nevertrump campaign, which seeks to keep the billionaire out of the White House.

Such is the resentment towards Trump, many are now readying themselves to vote for the Democrats despite being lifelong Republican supporters.

Dennis Jordan tweeted: 'I've been a lifelong loyal @GOP, since the day I could vote. Tonight, I have unregistered and now am an Independent.'

Lachlan Markay posted a photograph on Twitter of his District of Columbia voter registration card burning.