The Tesco chief executive, Dave Lewis, has announced his surprise departure after five years at the helm of the UK’s biggest retailer.

The former Unilever executive said he had completed the turnaround plan he set in train following the company’s 2014 accounting scandal and would leave the company in a “position of strength”. He will be succeeded next summer by Ken Murphy, a Boots lifer who most recently was the chief commercial officer of its American parent Walgreens Boots Alliance.

“My decision to step down as group chief executive is a personal one,” said Lewis. “I believe the tenure of the chief executive should be a finite one and that now is the right time to pass the baton.”

The 54-year-old made it clear that he did not have another job to go to and was “not discussing options with anyone”. “I want to take some proper time out with my family and recharge my batteries,” he said. “You pass this way only once.”

The Tesco chairman, John Allan, said that it was with regret that he had accepted Lewis’s resignation. “Dave has done an outstanding job in rebuilding Tesco since 2014 and he continues to have unwavering support from the board,” Allan said.

Murphy, an Irishman, studied at University College Cork, before going on to qualify as an accountant. Allan described him as a seasoned leader with “deep commercial, marketing and brand experience within retail and wholesale businesses”.

The announcement of Lewis’s departure came as Tesco reported a better-than-expected first-half operating profit before one-off items of £1.41bn, a rise of 25.4%. The retailer said it had made a strong start to the year and was well positioned to be highly competitive in a challenging market.

Shore Capital analyst Clive Black described Lewis as “the bloke that saved Tesco which should go down as an enormous achievement in British retail history”. He said Lewis’s departure would be a loss to the group leaving Murphy with “big shoes to fill”.

Independent retail analyst Nick Bubb said Lewis was able to say he was leaving with his “mission accomplished” but questioned why Tesco had chosen an external candidate to succeed him.

“What Dave Lewis will do next is unclear but the bigger question is why Tesco overlooked an array of internal candidates and have gone outside and appointed Ken Murphy from Boots (aka ‘Ken who?’) as the new boss,” said Bubb.

In issuing a separate subpoena last week as part of the impeachment inquiry, the chairmen of three committees made it clear stonewalling would be considered obstruction of Congress.

The expanding Ukraine scandal is threatening to engulf one of the president’s most loyal enforcers. Pompeo received a subpoena from the committees to turn over documents related to the Ukraine investigation. He said he would respond by the deadline of 4 October.

It also emerged this week that Pompeo participated in the July phone call in which Trump, having frozen military aid, pressed the Ukrainian president, Volodymyr Zelenskiy, to investigate baseless allegations against the former vice-president Joe Biden and his son Hunter.

The three committee chairs said on Tuesday that if the report of Pompeo being on the call were true, he “is now a fact witness in the House impeachment inquiry. He should immediately cease intimidating department witnesses in order to protect himself and the president.”

The secretary of state began a four-nation tour of Europe on Tuesday in Italy. He was accompanied by the former White House adviser Sebastian Gorka, who has a radio show and is travelling as a member of the press. Gorka is a hardline nationalist and former editor for the far-right Breitbart News. He has vehemently denounced the impeachment inquiry.

Trump is also defying political norms by accusing opponents of treason and making threats to the intelligence community whistleblower who raised concerns about the Zelenskiy call.

In an earlier round of volatile tweets on Tuesday, Trump had wondered why he was not “entitled to interview [and] learn everything about” the whistleblower, whose identity is protected by law.

The president also repeated his claim that the July call was “PERFECT”, adding: “This is just another Fake News Media, together with their partner, the Democrat Party, HOAX!”

But the Republican senator Chuck Grassley told reporters the whistleblower “ought to be heard out and protected” and requests for confidentiality should be respected.

The crisis has also cast a harsh spotlight on the attorney general, William Barr. The Washington Post revealed that he held private meetings overseas with foreign intelligence officials, seeking their help in a justice department investigation Trump hopes will undermine his own intelligence agencies’ conclusions about Russian interference in the 2016 election.

The countries concerned were Britain, Australia and Italy.

The Republican senator Lindsey Graham defended the strategy.

“Barr should be talking to Australia,” he told the Fox News host Sean Hannity. “He should be talking to Italy. He should be talking to the UK to find out if their intelligence services worked with our intelligence services improperly to open up a counter-intelligence investigation of Trump’s campaign.

“If he’s not doing that, he’s not doing his job. So I’m going to write a letter to all three countries … asking them to cooperate with Barr.”

Richard Painter, a former chief White House ethics lawyer under George W Bush, tweeted: “Impeach Barr and Pompeo after impeaching ⁦@realDonaldTrump⁩.”