Boris Johnson was grilled in the House of Commons a day after a revelatory report on Russia concluded that officials ignored meddling in elections.
Authors of the report had concluded that "serious questions needed to be asked” about why ministers didn’t look into the issue.
Labour Party leader Keir Starmer asked the prime minister why he had waited so long to release the Russia report.
Johnson said that Starmer was "motivated by a desire to undermine" the Brexit referendum, stating that the Labour leader was only interested in suggesting "Russian interference was somehow responsible for Brexit."
"The people of this country didn't vote to leave the EU because of pressure from Russia or Russian interference. They voted because they wanted to take back control of our money, of our trade policy, of our laws," the prime minister said.
Johnson also claimed that there is "no country that is more vigilant" in protecting against Russian interference.
The UK parliament's influential Intelligence and Security Committee report said it was "difficult to prove" allegations that Russia sought to influence the referendum but but said it was clear the government “was slow to recognise the existence of the threat".
PMQs were followed by an urgent question to the Home Office over the Russia report.
Home office minister James Brokenshire responded that the government "categorically rejects any suggestion that the UK actively avoided investigating Russia."
"Our world class intelligence and security agencies continue to produce regular assessments," Brokenshire said.
Labour party MP Nick Thomas-Symonds said the report showed the government "took a conscious decision not to look at all" at Russian interference. Thomas-Symonds said the Prime Minister had failed to respond to this during the questions.
Brokenshire said he would not take lectures from the Labour party and said the Conservatives have a good record on national security.
The government does not believe it was necessary to hold "a specific retrospective inquiry" because successful interference would "emerge through our existing processes".
Starmer and Johnson also had a back and forth about Russia Today broadcasts, with Johnson stating that former Labour leader Jeremy Corbyn had appeared on the program.
Starmer said that the party now has a new leader and that no one has appeared on the Russian television network since he has been leader.
This was the last PMQs before the summer holidays - Johnson also spoke about single market legislation and financial support for potential local lockdowns amongst other topics.