In the Commons, John Major side- stepped a call by Mr Ashdown to support the proposal by the United Nations High Commissioner for Refugees for 'safe areas'.
The Prime Minister said the UN wanted the areas agreed by the warring sides. But last night Mr Ashdown wrote to Mr Major insisting the UNHCR was proposing to declare the safe areas in Zepa and Gorazde.
Later, Mr Ashdown told the Independent that the protection of Muslim areas could be carried out under the existing UN mandate to protect life, which included the right to return fire if attacked.
Mr Major - after a renewed demand by John Smith, the Labour leader, for an ultimatum and possible air strike on Serbian supply lines - said no action could be carried out unless by the UN with Russian support.
'I see no other circumstances in which it would be practicable to expect any other military authority to be there and succeed. Any action other than in those circumstances would be unacceptable and unlikely to be agreed. I stress, that would be a very grave step to be taken and it is not yet the policy of any of the countries of the UN.'
But Mr Ashdown said that if the UN refused to act, Nato should send more troops to defend the Muslims: 'It's got to the stage where it's double or quits. Either we are going to do it or we aren't. If we are not we should get out,' he said.
He warned that unless the UN was prepared immediately to take tougher action, the Croats would think 'Muslim territory is up for grabs' and would open a third front.
Croat attacks against the Muslims would bring the UN humanitarian effort to a standstill, because the besieged Muslim towns were being aided through Croat territory.
'Unless the UN is prepared to show . . . that it is prepared to defend Muslim territory, there is a catastrophe for the Muslims lying just around the corner and extreme danger for the UN. The UN could be forced to leave with its tail between its legs.
'What I would be prepared to do is what General Morillon (Philippe Morillon, UN commander in Bosnia) wanted to do in Srebrenica to protect the Muslims. I would be prepared to do that in Zepa and Gorazde.'
Mr Ashdown, who has toured Serbian supply lines, strongly opposed the calls for air strikes on Serbian supply lines by Baroness Thatcher, Lord Owen and John Smith.
'Nobody who has seen the position on the ground would recommend this action. There are squadrons of tanks . . . right up to the front. They don't need supplies from Serbia. These people are past masters at throwing up new tracks through the forest and across mountains.
'The river bridges would have to be destroyed but they are in densely populated Serbian towns. Even with smart bombs in the Gulf, only 40 per cent hit their targets. It would be certain to cause civilian casualties.
'There would be nothing better calculated to strengthen the Serbs in worsening this conflict. I would support arming the Muslims - and I don't advocate it - before I supported that.'
Mr Ashdown said the West's response had been 'chaotic and timid; it has been unco-ordinated and unclear; there has been a military aim - to deliver humanitarian aid - but there has never been a political strategy. That is a recipe for disaster.'