Clare Short is set to resign from the Cabinet unless the prime minister secures a second resolution sanctioning force in Iraq.

Warning that it is time to put her "cards on the table", the international development secretary said she will quit government if Tony Blair goes to war without the backing of the UN.

"If there is not UN authority for military action or if there is not UN authority for the reconstruction of the country, I will not uphold a breach of international law or this undermining of the UN and I will resign from the government," she told the BBC last night.

"I think it's time for cards on the table. People are speculating and making all sorts of statements about my intentions. I think I owe it to my colleagues in the government and members of the Labour Party to just be truthful about my position. It's the time to say what my intentions are."

Short said the prime minister's actions in relation to Iraq had been "deeply reckless".

Her intervention is a severe blow to the prime minister who yesterday night faced his first government resignation over the looming Iraq war.

Andy Reed, the parliamentary private secretary to environment secretary Margaret Beckett, announced his decision to quit amid increased speculation that Britain will not secure a second UN resolution.

"I will set out my full reasons for resigning, but I fully support the prime minister in his attempts to find a peaceful solution to the Iraq crisis through the UN route and do not want to do anything that undermines that effort at this stage," said Reed on his website.

In an interview with today's Guardian, Chris Smith, the former culture secretary, says "there are very few people on the Labour benches who are happy with the position that we find ourselves in".