Outspoken MP Greg Mulholland has called on pubcos to commit to passing on any future duty cuts to licensees.
He has written to chief executives of Punch Taverns and Enterprise Inns, as well as the British Beer & Pub Association, as chairman of the All-Party Parliamentary Save the Pub Group.
The letter says the Group backs the trade's Axe the Tax campaign and wants duty on beer to be lowered.
"As part of this argument however, it is essential that we convince Government that if action is taken to reduce beer duty, then this will be passed on to licensees and to customers.
"As a result we are asking all pub operators to give a guarantee that any reduction in duty will be passed on to their licensees. "
The Liberal Democrat MP for Leeds North West said in a statement: "We must all keep campaigning for lower beer duty and I hope the message is now getting through to ministers.
"But it is also essential that the pub companies and pub owning breweries guarantee they will pass any duty cut on to licensees so it actually brings down the price of a pint in pubs. Otherwise it might prop up some faltering companies but would not save pubs.
"With this guarantee in place, we can put more pressure on the Chancellor to act because we could then be sure that a cut in beer duty would really help pubs at this very difficult time."