Dan Jarvis has told MPs that the Russian oil tanker seized on Sunday had been monitored for several days as he deflected suggestions that its seizure had been ordered by a prime minister under pressure after the resignation of John Healey last week.

The new defence secretary, flanked supportively in the Commons by the chancellor, Rachel Reeves, said the Smyrtos had been “closely tracked” as he faced a question from the Conservatives as to why the capture took place over the weekend.

Surveillance of the tanker – which was carrying crude oil from Russia to India – had gone on “in the days” leading up to its seizure by Royal Marines, Jarvis said, “on the grounds it was suspected of using a false flag” of Cameroon.

It was captured 25 miles south of the Isle of Wight in the small hours of Sunday, when marine commandos roped down on to the deck of the 244-metre vessel. The ship was captured in “a matter of minutes”, the defence minister said.

The captain of the ship, Indian national Ajay Pant, 38, was charged with breaching sanctions offences and will appear at Southampton magistrates court on Tuesday, the National Crime Agency said, while 24 crew members from Georgia and India remain on board.

Questions had been asked about the timing of the seizure, which took place three days after Healey resigned on Thursday in an acrimonious row about the money No 10 and the Treasury were willing to commit to defence by 2030.

“You have been unable, and the Treasury has been unwilling, to commit the resources that the nation needs to defend the country,” Healey said in his dramatic resignation letter, which caught Westminster by surprise.

The Labour veteran had quit after the Treasury had only offered £13.5bn to plug an £18bn gap in the defence investment plan (Dip) – while Starmer had only been willing to lift defence spending to 2.68% of GDP by 2030, below a Nato target of 3.5% by 2035.

Reeves sat next to Jarvis through the first half of his statement, his first Commons outing as defence secretary. “The chancellor is someone who cares very deeply about our national security, and that is why she is here to lend that support,” Jarvis said.

But there was no further update on the status of the Dip from Jarvis, other than for the newly appointed defence minister to repeat it would be published before the Nato summit in Ankara in early July.

James Cartlidge, the shadow defence secretary, asked Jarvis why the vessel was seized this weekend, when dozens of other shadow fleet vessels had crossed into UK waters after Starmer had warned in March that tankers carrying Russian oil under sanctions could be at risk of capture.

“Until this weekend, almost three months after the policy was announced, no boarding of these vessels had taken place. What changed on the specific action that took place?” the Conservative MP asked.

The defence secretary said that the principle of a seizure had been “the result of weeks of military and operational planning” but he would not say if the UK would try to sell the 100,000 tonnes of crude oil on board the Smyrtos to raise funds for Ukraine’s defence against the continuing Russian invasion.