LONDON (AP) — U.K. Defense Secretary John Healey resigned unexpectedly on Thursday, criticizing the government for inadequate military spending amid increasing threats. This resignation presents another challenge for Prime Minister Keir Starmer, who faces pressure from Labour Party members to step down.

In a letter to Starmer, Healey stated that the Defense Investment Plan does not meet the necessary requirements during such a critical time. The publication of the plan was delayed due to disagreements between the Defense Ministry and the Treasury.

Al Carns, a decorated war veteran and prominent junior minister, resigned shortly after Healey. Carns expressed dissatisfaction with the insufficient investment and hinted at possibly challenging Starmer’s leadership.

Starmer wrote to Healey, expressing regret over his resignation while maintaining that the funding plan would ensure sustainable and fair increases in military spending. Late Thursday, Dan Jarvis, former security minister, was appointed as the new defense secretary.

Starmer committed to raising U.K. defense spending to 2.5% of GDP by 2027 and 3% by 2035. Many military officials believe this timeline is too slow. In his resignation letter, Healey said the Treasury’s spending plan projects a rise to 2.68% in 2030, with a 2.6% increase next year. Healey argued that these levels are insufficient given current defense demands and military commitments.

General Richard Barrons criticized the government’s refusal to fund its defense review, asserting it weakens the U.K.’s NATO standing and increases vulnerability. The government claimed it is providing the largest defense spending boost since the Cold War.

Healey, regarded as a capable minister, played a crucial role in supporting Ukraine and forming an international coalition for regional security. He also worked to keep the Strait of Hormuz open for shipping amidst the Iran conflict.

Under pressure from the U.S. to increase military spending, the U.K. is attempting to reverse military decline amid Russian aggression. NATO Secretary-General Mark Rutte, informed of Healey’s resignation, acknowledged the difficulties in balancing defense investments with other expenses.

Healey’s departure may heighten speculation about Starmer’s potential ousting. Starmer faces challenges in reconciling differences over defense spending, with some party members advocating for his resignation.

Olivia O’Sullivan of Chatham House suggested that the resignation undermines Starmer, despite his solid record in defense matters. Greater Manchester Mayor Andy Burnham might contest Starmer’s leadership, given an upcoming special election.

Justin Crump, a security consultant, criticized the government’s handling, citing a lack of control and clarity.

Contributors to this report include Mark Carlson in Brussels, and Pan Pylas and Brian Melley in London.