Secretary of state for work and pensions Esther McVey has resigned from the government.
McVey took on the role in January this year after a cabinet reshuffle and has been in post for just 312 days.
She resigns after a tumultuous cabinet meeting over a draft agreement for Britain's withdrawal from the European Union, which took place yesterday.
In a resignation letter to Theresa May, McVey said: "We have gone from no deal is better than a bad deal, to any deal is better than no deal. I cannot defend this, and I cannot vote for this deal. I could not look my constituents in the eye if I were to do that. I therefore have no alternative but to resign from government."
She said the deal put before Cabinet yesterday does not honour the result of the EU referendum, and does not meet the tests May set out from the outset of her premiership.
The proposals will "trap us" in a customs union, "bind the hands" of future governments in pursuing genuine free trade policies, by handing over £39bn to the EU "without anything in return", she said.
She added it has been a "huge honour" to serve as secretary of state.
Earlier this morning I informed the Prime Minister I was resigning from her Cabinet pic.twitter.com/ZeBkL5n2xH— Esther McVey (@EstherMcVey1) 15 November 2018
Royal London director of policy and former pensions minister Sir Steve Webb said McVey's successor would face many competing demands.
"As a result, they will have little time to shape pensions policy.," he added. "With this in mind, the key people shaping pensions policy will remain Treasury and the Department for Work and Pension's (DWP) pension minister [Guy Opperman]."
Lincoln Pensions managing director Alex Hutton-Mills said the resignation and Brexit negotiations more generally could have a significant impact for defined benefit (DB) schemes.
"The sponsor covenant has to continually underwrite any ongoing funding volatility and now, more than ever, trustees should have actionable contingency plans in place to protect member benefits," he said.
"In addition, Esther McVey's resignation could disrupt DWP's involvement in the impending consultation on DB consolidation. Given the potential for consolidation to improve member security in certain circumstances, any further delay would be unfortunate."