A former Army major has overturned a 24,000 Conservative majority to win the Tiverton and Honiton by-election.
Newly elected Liberal Democrat MP Richard Foord served in the Army for a decade.
He received three campaign medals for service in Iraq and the Balkans and also served as a UN peacekeeper in Kosovo.
Richard Foord grew up in the West Country and served for 10 years in the Army after graduating from Sandhurst, reaching the rank of major.
He received three campaign medals for service in Iraq and the Balkans.
Mr Foord also served as a UN peacekeeper in Kosovo.
In June, after being announced as the Liberal Democrat candidate for the then-upcoming Tiverton & Honiton by-election he told UK Liberal Democrats about his pride in his service.
He said: "I was proud to serve my country in the Armed Forces for 10 years and now want to serve my community in Westminster."
The former Army major has also raised thousands of pounds for charities, including running the London Marathon for the Royal British Legion.
In a tribute to the Queen on her Platinum Jubilee, he described his time serving in Her Majesty's Armed Forces as "one of the proudest periods of my life".
Mr Foord described his victory as "absolutely staggering".
Speaking just after the vote was declared early on Friday morning, he said: "This result is absolutely staggering, we weren't expecting a win, let alone a win of this scale."
He added: "We have been noticing that momentum has been shifting from the Conservatives to the Liberal Democrats over the course of this campaign and particularly in recent days because of the very positive agenda we were setting out.
"But this is absolutely seismic and we have overturned the biggest majority in parliamentary by-election history and it really sent a very clear message to this Government."
The Liberal Democrats hope the victory is part of a resurgence of the party in the region following their disastrous election results of 2015 when they lost all of their seats in the South West, a region once considered a party stronghold, particularly during the leadership of the late, former Royal Marine Paddy Ashdown.
Call for Boris resignation
Prime Minister Boris Johnson said he would "keep going" after his authority was dealt a series of blows by the double by-election defeat – Labour also took Wakefield in a second by-election – which triggered the resignation of Cabinet minister and Tory party chairman Oliver Dowden.
In his victory speech, Mr Foord called on Boris Johnson to take "the only decent course of action left open" and resign.
He said: "Our country is crying out for leadership.
"I served as an officer in the British Army for 10 years, Mr Johnson, and I can tell you that leadership means acting with decency and integrity.
"It means keeping your word. It means setting an example and putting other people's needs before your own.
"I served alongside friends who personified these values and laid down their lives in the service of their country.
"And yet, your behaviour, Mr Johnson, makes a mockery of leadership, by any measure you are unfit to lead.
"The people of Tiverton and Honiton have told you tonight that enough is enough, they demand a change.
"The only decent course of action left open to you is to heed their call and resign."