Liberal Democrat leader Nick Clegg has urged Gordon Brown to boycott this summer's Olympic opening ceremony in protest at China's human rights record.
He said the prime minister could not attend the event in Beijing "in good conscience", given the country's crackdown on protests in Tibet.
In a letter, Mr Clegg said Mr Brown must "take a stand for human rights".
The prime minister has ruled out a boycott. The Olympic torch passes through London this weekend.
Tennis player Tim Henman, athlete Kelly Holmes and cricketer Kevin Pietersen will be among the 80 bearers.
'Reconsider'
The torch will be carried 31 miles through 10 London boroughs and welcomed by Mr Brown outside 10 Downing Street.
Protests against China's reaction to demonstrations in Tibet are expected along the route, as are protests by Chinese students in support of the Games.
""Serious concerns cannot be swept under the carpet for the sake of ceremonial duties.""
In his letter to the prime minister, Mr Clegg said: "I understand that on Sunday you will welcome the Olympic torch to London.
"I urge you to reconsider your participation in this event and indeed any future events to mark the Beijing Games, including the opening ceremony on the 8 August.
"Recent events in Tibet, broken promises over media freedoms, ongoing human rights abuses and intransigence over the humanitarian catastrophe in Darfur demand a response.
"Serious concerns cannot be swept under the carpet for the sake of ceremonial duties."
Downing Street sources say the prime minister's welcoming of the torch does not mean is any less concerned about the situation in Tibet, but is mindful that the Dalai Lama has advised that countries should not boycott the Beijing Games. However, there is still concern in government circles about the level of protests that may be seen on Sunday and fears they may damage London as well as Beijing.
Tibet's government-in-exile, based in India, says about 140 people were killed in the crackdown on recent unrest by Chinese security forces. Beijing disputes this, saying 19 people were killed by rioters.
The Olympic torch will go through 20 countries before being carried into the Beijing Games' opening ceremony.
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