David Cameron announced on Friday his plans to step down as Prime Minister by October after the UK voted for Brexit.

The departure of Cameron, who has been Conservative Party leader since 2005 and Prime Minister since 2010, will spark a Tory leadership contest.

Fevered speculation as to who will succeed the Prime Minister has already begun in earnest. Let's take a look at the runners and riders.

The former London mayor is tipped by many to take over the reins at Downing Street, especially after his championing of the Leave cause, which has installed him as the bookies' favourite.

Johnson was widely considered a possible future PM long before that.

But it's no sure thing for the Uxbridge and South Ruislip MP, who may not be as popular with his own MPs - a majority of whom voted Remain - as he seems to be with his party's Eurosceptic grassroots.

Pro-Remain Tory MP Alan Duncan has also raised doubts over the strength of Johnson's grassroots support, remarking on Saturday: "Do not necessarily assume that he is the darling of the Conservative Party activists."

He added: "A lot of them have loved the notoriety and the excitement. But actually, once you scratch the surface a little bit and ask the second question, a lot of them don't want a permanent ride on the big dipper."

A strong favourite, perhaps, but not a dead cert.

Theresa May, the longest-serving Home Secretary in over a century, could very well emerge as the favoured 'Stop Boris' candidate if the disdain for Johnson shown by pro-EU parliamentarians like Alan Duncan is anything to go by.

Despite backing Remain, May flirted with the other side by arguing that Britain should leave the European Convention on Human Rights (ECHR) "regardless of the EU referendum".

She is the bookies' second favourite after Boris, and is seen as having kept her hands fairly clean during the EU referendum by mostly avoiding the limelight.

May has more experience in her Home Office brief than anyone else in the UK cabinet besides Cameron himself and Chancellor George Osborne.

The value of a perceived 'safe pair of hands' in the aftermath of such a tumultuous event should not be underestimated.

The 43-year-old work and pensions secretary was only appointed to the position in March, but what Stephen Crabb lacks in Cabinet experience he makes up for in image, say his advocates.

He is young, considered a rising star, and comes from a working-class background, brought up on a council estate by his single mother.

The success of Scottish Tory leader Ruth Davidson is thought by some a sign that the UK party needs leaders from more relatable backgrounds.

Indeed, Davidson herself has been mooted as a potential Prime Minister at some point in the future.

For her part, she considers Crabb a "political soulmate" and has said she would "find it very hard to vote for anyone else" should the Welsh MP enter the leadership fray.

Andrea Leadsom, who featured in TV debates for Leave alongside Boris Johnson, is thought to have had a good referendum.

The energy minister backed Cameron to stay on regardless of the referendum result, but has been installed as the bookies' fourth favourite.

She squared off on live television against Remainers in her own party during the campaign, clashing with Department of Energy and Climate Change colleague Amber Rudd and dubbed a liar by Ruth Davidson.

The 53-year-old South Northamptonshire MP has also been linked to the top job at the Treasury should George Osborne leave his post.

But what of the Chancellor himself?

Long-touted as a possible successor to Cameron, his leadership chances are seen to have been dramatically reduced now the Remain cause he hitched his wagon to has been defeated.

While Cameron led the news on Friday following his resignation, his ally Osborne has stayed out of the spotlight, no doubt planning his next move.

ITV's political editor Robert Peston has speculated the Chancellor's experience could yet keep him in a top job, tipping him for future Foreign Secretary, saying: "Someone is going to have to negotiate the UK's extrication from the EU, and no one in the Tory Party has Osborne's technical expertise.

"If Osborne can't be leader, at least this time, he would be the rational choice for Foreign Secretary of whoever comes after Cameron."

Justice secretary and former education secretary Gove was Boris Johnson's senior ally in the Leave campaign.

He has ruled himself out from the top job in the past, most recently in May, but is the bookies' third favourite for to succeed his long-time friend Cameron.

More likely, some feel, is that he will back Boris and help him lead a 'Brexit government', no doubt taking a senior post in the process.

Nicky Morgan, the ambitious pro-Remain education secretary, is currently long odds to succeed Cameron.

The Loughborough MP may be testing the waters for a run at the top job, commenting on the BBC's World at One that "it would be good to see a woman in the final two" of any Conservative leadership ballot.

Her strong support for staying in the EU could cost her votes in any leadership contest.

Perhaps a more likely long shot would be pro-Leave employment minister Priti Patel, or even anti-EU former defence secretary Liam Fox.

Whoever becomes the next Conservative Party leader, it is widely thought that a snap General Election will be called afterwards sooner rather than later.

Labour's shadow chancellor John McDonnell has said his party are "gearing up" for a snap election by the end of the year.