Sterling, FTSE 100 gain as UK PM Boris Johnson steps down

Sterling moved higher against the U.S. dollar Thursday after Prime Minister Boris Johnson announced his departure from office.

The pound traded $1.1977, up 0.37%, at 12.30 p.m. U.K time.

Meanwhile, the FTSE 100 stock index was trading around 1.2% higher following Johnson's speech.

In bond markets, the 10-year U.K. government bond yield rose to 2.124%.

It comes after more than 50 MPs resigned from Johnson's government within 48 hours.

The prime minister has been under increasing pressure since Tuesday, when two government heavy weights, the health and finance minister, resigned from office. This led to a cascade of departures from government.

Roger Jones, head of U.K. equities at London and Capital, told CNBC there was "some relief" in currency markets because it was looking as if "this is not going to be long and drawn out with the prime minister refusing to go and ultimately causing a policy stalemate."

The first resignations began in earnest on Tuesday, but Johnson clung to his position, appointing replacement ministers. However, even some of the new appointees have now told him that he needs to go.

Newly appointed finance minister Nadhim Zahawi said in a letter "you know in your heart what the right thing to do is, and go now."

Market players are already wondering what the latest political developments will mean for the U.K.'s economy.

"If Boris Johnson resigns as UK Prime Minister later today, as the media is now reporting, then it's possible that fiscal policy may be a bit looser and monetary policy may have to be a bit tighter," Paul Dales, chief U.K. economist at Capital Economics, said in a note.

Boris Johnson came into power back in July 2019, replacing Theresa May. Johnson called for a snap election later that year, which saw him obtain a parliamentary majority.

Thursday's developments follow a premiership plagued by scandals and allegations of misleading the public, but the final straw for many MPs involves Conservative lawmaker Chris Pincher. The former deputy chief whip was suspended last week amid accusations that he drunkenly groped two men at a private members club.

Johnson on Tuesday apologized for appointing Pincher deputy chief whip — a senior party role — despite knowing of an investigation into his behavior in 2019.

The revelation that Johnson knew of the misconduct allegations prompted the resignations on Tuesday of Chancellor Rishi Sunak and Health Secretary Sajid Javid.

In a resignation speech to parliament Wednesday, Javid, also a former chancellor, said "treading the tightrope between loyalty and integrity has become impossible in recent months."

Johnson narrowly survived a confidence vote from Conservative MPs last month, but many of those previously backing his leadership have now abandoned their support.

--CNBC's Elliot Smith contributed to this report