Two Conservative MPs have voiced concern about the scale of George Osborne’s proposed cuts to tax credits, raising the possibility of a wider backbench rebellion.

Guto Bebb, MP for Aberconwy, and Andrew Percy, MP for Brigg and Goole, suggested there needed to be more protection for the poorest who face losing government support despite working hard in low-paid jobs.

Both MPs voted for the government’s welfare reform and work bill at its second reading in July, but their decision to speak out about the potential impact of the cuts will be a warning signal to Osborne not to take the support of backbenchers for granted.

Percy told the Times: “We need to be very careful when making changes to the tax credits of those already in the system that those changes are tapered and offer protection to those affected. Many of these people are doing exactly what the government has asked them to do: get up in the morning and go to work.

“Not everyone has either the skill base, or necessarily lives in the right area, to be able to just go and get a better-paid job. The changes to the minimum wage and personal tax allowances will help many, but not all, of those on low wages affected by tax credit changes. Change is needed, but in the process we mustn’t end up harming those who are trying their best and working hard.”

Bebb said: “While the introduction of a national living wage was rightly lauded, the changes to tax credits have been somewhat under-scrutinised. The changes are both eye-wateringly painful to those affected, but also reverse a key policy platform of the last five years, namely making work pay.”

Their unease is shared by some others within the Conservative party. One Tory MP told the Guardian there was particular disquiet about the possibility that the cuts could lessen the incentive for people to work, which is exactly the opposite of the government’s stated aim on welfare.

He said he expected there would not be any full-blooded rebellion when the welfare bill returns to the House of Commons, but some MPs would like to see a slower implementation of the £5bn cuts to tax credits and more consideration of the consequences.

There is also concern that the decision to slash tax credits is a reversal of David Cameron’s position before the election that he did not want to cut child tax credits. Michael Gove, the justice secretary, also signalled before the election that there would be no cuts to tax credits, saying: “We’re going to freeze them for two years, we are not going to cut them.”

However, in the budget, Osborne announced plans to reduce income thresholds in tax credits and work allowances, freeze the benefits for four years and limit child tax credit to the first two children, as part of plans that will reduce welfare spending by a cumulative £46bn over five years.