David Cameron has been urged to consider sending the RAF to carry out emergency food drops to help starving Syrians.
Shocking reports emerged of civilians in a besieged town dying through lack of food while others were eating dirt and rubbish.
Former Liberal Democrat leader Paddy Ashdown and Labour MP Jo Cox have written to the Prime Minister warning that an agreement to get aid into the stricken areas may be an "empty gesture" and called on him to consider sending in help.
It comes as Queen Rania of Jordan called for a "new approach" in dealing with refugees that would work in Europe and "address the needs of refugees" during talks in Downing Street.
In a meeting with Mr Cameron, she urged the international community to take "bolder measures" to deal with the refugee crisis, which has seen displaced Syrians flee to neighbouring countries, including Jordan.
Queen Rania added: "I would like to thank the British people for their generosity and support and compassion in dealing with this, what could be one of the worst humanitarian crises that we face in our time."
A conference is being held in London next month to secure funding for Syrian refugee camps in Jordan, Lebanon and Turkey.
Mr Cameron has said the reports coming out of Madaya "are heart-wrenching" and underline the summit is "vital".
UN officials said the Syrian government has agreed to allow aid into Madaya where more than 30 people have reportedly died of starvation or been killed trying to escape in the past month.
The town, which lies north west of Damascus near the border with Lebanon, has been under siege by the Assad regime and its Hezbollah allies since July.
But Lord Ashdown and Ms Cox warned that the deal did not go far enough and urged the Government not to "sit by and watch this happen."
They wrote: "We urge you to push the UN, in particular the Office for the Coordination of Humanitarian Affairs, to be far bolder in its aid delivery and stop asking unnecessary permission from the Syrian government.
"In the case that the UN continues to be denied access to these besieged areas by the Assad regime, the UK should strongly consider air dropping aid to those communities at risk of starvation. In some of these areas, the RAF is already flying anti-Isis missions, and if necessary this is something we should press our European partners to support."
Downing Street later said the Prime Minister and Queen Rania had discussed the need for a "comprehensive approach" to the Syrian humanitarian crisis.
They agreed that "as well as substantially increasing humanitarian aid, countries must seek to address the longer term needs of refugees through education and employment, to enable them to return to Syria and rebuild its economy once the conflict has ended".
According to Number 10, they both stressed that education was "vital" and reiterated their commitment to ensuring greater access to schooling for all children in Syria and neighbouring countries.