Alwin Oliver, the vice-chairman of Portsmouth & District Private Landlords' Association, said city council plans to extend licensing to smaller homes risked them being converted into more expensive 'super HMOs'.
'Our prediction is a significant increase in community landlords exiting the market, or repurposing for family let and a demand-led net growth in company landlords creating more super HMOs,' he said. 'We have members who provide both. It is for the city to decide which is the preferred model.
He added: 'Overall, these proposals will push up rents - the median rent will jump 50 per cent.
'The people who will suffer are those on benefits who currently struggle to survive in the cheapest HMOs as these will be pushed right out of the market.
'Add to this the loss of community landlords and their positive input to the local economy, being replaced by corporate landlords who add little to the local economy and the net effect of additional licensing will be severely negative on the city, even before you consider the crisis of removing homes from the 1,000-1,500 people most in need of affordable housing in the city.'
The city council is considering bringing in the new licensing system to cover three and four-bedroom HMOs after data showed serious faults reported to the council were disproportionately found in them.
Under the existing rules, just over 1,200 of the largest homes are required to be licensed. The council has estimated there are about 6,000 HMOs in the city.
The council consultation, which has attracted almost 1,000 responses according to cabinet member for housing Darren Sanders, had been due to end last week but has been extended until today due to technical issues.
'We had this licensing system in place in the city from 2013 until 2018 and there was no significant increase in rent at that time,' Cllr Sanders said. 'There are a range of factors with the cost of living at the moment which are doing that.
'What we want to do with additional licensing is to make sure that everyone who is renting a room in an HMO gets somewhere that's up to a good standard.'