The service on Sunday will be open to people of all faiths and none and will take place at St Thomas Church in the city centre just metres from where Sergei and Yulia Skripal were found.
Last week, Johnson faced accusations of misleading the British public after he said in a televised interview that scientists at the government's military laboratory, Porton Down, had told him the nerve agent definitely came from Russian Federation.
The toxin attack prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies in Europe and the United States sided with May's view that Moscow was either responsible or had lost control of the nerve agent.
Yulia, 33, has been discharged from Salisbury District Hospital, Christine Blanshard, medical director of the hospital, told reporters on Tuesday and her father could be discharged in due course.
"The text has been composed in a special way so as to support official statements made by British authorities and at the same time to exclude every possibility of Yulia's contacts with the outer world - consuls, journalists and even relatives", the embassy said. "This is not the end of her treatment, but marks a significant milestone".
He soon reaches the microbiology department, which he says is behind a "securely locked" door with a sign saying "do not enter, stay away". Britain says that means only a state with a sophisticated laboratory could have manufactured it.
Russian Federation said it would consider any secret resettlement of the Skripals as an abduction of its citizens.
Novichok is a military-grade nerve agent developed by Russian Federation from the 1970s onward, however the country's president Vladimir Putin has warned that it could be produced in up to 20 countries.
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Russia, which denies it was behind the attack in Salisbury, called the allegations an "anti-Russian campaign".
The Novichok class of nerve agents was developed in the Soviet Union toward the end of the Cold War, and Britain says it has evidence Russian Federation has continued to manufacture Novichok agents in the last decade. But it said it agreed with the UK's findings on the identity of the nerve agent. She said this had resulted in sickness and hallucinations.
"They were asked to leave the hospital and did so", a spokesperson said.
Despite initial doubts that they would not survive, Yulia Skripal was released from hospital to an undisclosed location on Monday.
Sergei Skripal, who was recruited by Britain's MI6, was arrested for treason in Moscow in 2004.
The Skripals were found collapsed on a park bench in Salisbury, Wiltshire, on March 4th. He has British citizenship.