Russia's RT and Sputnik news agencies have been banned from attending a conference on media freedom in London for playing an "active role in spreading disinformation".
Around 60 ministers and 1,000 reporters and members of civil society are due to attend the event this week.
But the UK Foreign Office refused accreditation to both RT and Sputnik.
The Russian Embassy called the decision "direct politically motivated discrimination".
A spokeswoman for the Foreign Office, which organised the conference, said: "We have not accredited RT or Sputnik because of their active role in spreading disinformation. While it's not possible to accommodate all requests for accreditation, journalists from across the world's media are attending the conference, including from Russia."
The Russian Embassy said it had complained to the Foreign Office over the decision, accusing the UK of a "months-long smear campaign" against RT.
RT said in a statement: "It takes a particular brand of hypocrisy to advocate for freedom of press while banning inconvenient voices and slandering alternative media."
Sputnik said: "Our goals are clearly indicated in our charter and spreading disinformation is not one of them."
RT, which is backed by the Russian state and was formerly known as Russia Today, has in recent years increased its coverage of UK and US news, attempting to position itself as an alternative to mainstream media outlets in both countries.
In December, the channel was found by UK broadcast regulator Ofcom to have committed seven breaches of the UK's broadcasting code during its coverage of the Novichok poisoning of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury.
Ofcom said RT failed to give due weight to a range of voices and called the breaches "a serious failure of compliance".
The Foreign Office said the international human rights lawyer Amal Clooney would attend this week's event as Britain's special envoy on media freedom.
The conference will also be attended by government ministers, members of the diplomatic community and academics.