Police officers could be replaced by private guards to cover some aspects of security at the Houses of Parliament.
The Metropolitan Police force currently provides all security services for the Commons and the Lords.
In a cost-saving move, airport-style security checks at the entrances will be put to tender - although the Met will still provide armed officers.
Labour MP Barry Sheerman said he was concerned private security companies would rely on agency staff.
But parliamentary officials said there would be no downgrading of security as a result of any changes.
The decision to split the contract was made by a joint committee of MPs, peers and security experts.
It is an attempt to find savings in the £24m parliamentary security budget.
The Met will be free to bid for the scanning and checking service alongside rival operators when the tendering process starts next year, so there could be no change to the current arrangements.
Referring to the possibility of agency workers being used, Mr Sheerman said: "Security comes from highly motivated professional staff who care about the place.
"At a time when security is a serious concern, we are making ourselves highly vulnerable to churn and weakening our relationship with the Met Police."
He also questioned Commons authorities' insistence that security at Parliament should not be discussed in public, claiming there was "seething resentment" among staff and MPs about what he described as a lack of consultation on the decision to split the security contract.
After Mr Sheerman raised concerns earlier this week, Commons Speaker John Bercow replied he was "very comfortable" with security arrangements of the House - but they would not be discussed on the floor of the chamber.
Imitation weapons, swords and knuckle dusters are just some of the items confiscated by police at public entrances to Parliament in the past few years.