The prime minister has insisted United Nations sanctions against Robert Mugabe are still possible, despite Chinese and Russian opposition.
This weekend financial sanctions against members of the Zimbabwean president's inner circle and an overall arms embargo were thrown out when Beijing and Moscow used their security council vetoes.
Both countries said they were not prepared to interfere in "national issues" while talks between Mr Mugabe and opposition leader Morgan Tsvangirai were taking place.
Speaking at his monthly press conference at Downing Street on Monday, Gordon Brown said the Chinese/Russian argument would only hold up as long as mediation talks continued.
He said that unless a breakthrough was achieved when the two countries would have to "face up to the facts that mediation had not succeeded".
"EU sanctions remain very important," the prime minister continued, "and the UN envoy is going ahead and will go to Harare.
"We have not only EU sanctions but also America too. Clearly, if mediation is not successful we reserve the right to return [to the security council]."
When pushed on the issue Mr Brown added that the stance of China and Russia, dubbed "incomprehensible" by foreign secretary David Miliband, was indefensible and completely unjustified.