MPs have questioned the ability of the Post Office's management to boost profits, a day after Royal Mail workers backed plans for strike action.
MPs said the UK needs to have a sustainable post office network
The trade and industry select committee outlined a number of worries it had about the postal network and called for the government to play a greater role.
Members urged management to show more "imagination and entrepreneurial flair in developing new areas of business".
Royal Mail-owned Post Office aims to shut 2,500 of its 16,000 outlets.
The closures would be made across the UK under modernisation plans.
The MPs called on the government to get involved and help boost the competitiveness of the network, and not just "manage its decline".
"The post office network cannot be left as it is, with low morale among sub-postmasters and the likelihood of significant numbers of closures which leave gaps in provision," the select committee said.
"We therefore welcome the government's determination to establish a comprehensive national network," it continued.
"However, we want that network to be sustainable, and we are yet to be convinced that either the government or Post Office has thought through what to do in the event of future closures," it added.
The MPs also said that there was a lack of a coherent plan "to make post offices more profitable and less dependent on continued public subsidy".
"Without this, there is a real danger that we will be faced with an inadequate network within a few years," they concluded.
The comments came a day after postal workers voted strongly in favour of taking industrial action over pay, in what would be the first national postal strike since 1996.
The union is in dispute with the Royal Mail over its 2.5% pay offer, and it is expected that about 130,000 CWU members will now stage walk-outs.