The South Cambridgeshire MP Heidi Allen has joined two other MPs in resigning from the Conservative Party to join the newly-formed Independent group in the House of Commons.
She was joined by fellow Conservatives by the Broxtowe MP Anna Soubry and the MP for Totnes Sarah Wollaston.
- Click to watch a report by ITV News Anglia's Russell Hookey
Explaining her decision to quit the Conservatives, Heidi Allen blamed Brexit and the Tory approach to welfare reforms.
Ms Allen, a member of the Work and Pensions Select Committee, said:" I can no longer represent a government and a party who can't open their eyes to the suffering endured by the most vulnerable in society, suffering which we have deepened whilst having the power to fix."
On Twitter, the former Conservative party chairman Lord Pickles, who was the MP for Brentwood and Ongar, said it was "expected, sad, and ultimately a big mistake for the individuals."
- Heidi Allen explains her decision to quit the Conservative Party to ITV News Anglia Jonathan Wills
The Independent group was formed on Monday after seven Labour MPs, including the Luton South MP Gavin Shuker, left the party.
An eighth, Joan Ryan, joined on Tuesday night and now the total with the former Conservative MPs joining is 11.
Heidi Allen was elected as the Conservative MP for South Cambridgeshire in the 2015 General Election succeeding former Health Secretary Andrew Lansley.
In 2015 she had a majority of more than 20,000 votes but that was cut to under 16,000 in the 2017 election.
She has been a vocal critic of government policy on welfare reform and universal credit as well as Brexit.
Prime Minister Theresa May said: "I am saddened by this decision - these are people who have given dedicated service to our party over many years, and I thank them for it."
Heidi Allen was an ardent campaigner for remaining in the European Union. She supports a People's Vote.
In the 2016 referendum, South Cambridgeshire was one of the few areas in the Anglia region to vote to remain in the EU.
In their resignation letter the three MPs said the final straw was the Government's "disastrous" handling of Brexit.
Heidi Allen is originally from Notton, Yorkshire and worked in business for 18 years for companies such as ExxonMobil and the Royal Mail. She was also the managing director of her family's manufacturing business.
She served as councillor in St Albans in Hertfordshire and has a degree in astrophysics from University College London