Theresa May has been accused of bringing the honours system into disrepute after handing a knighthood to a former minister known to be wavering on whether or not to support her Brexit deal.

Downing Street announced the award had been granted to John Hayes MP, a junior minister to May when she was the home secretary, on Friday afternoon.

“People will rightly look at this knighthood and wonder how it relates to the looming Brexit vote in the Commons,” said a spokesman for the Scottish National party leader, Nicola Sturgeon.

“That ‎brings the honours system into disrepute. But, more fundamentally, it exposes how broken the whole Westminster system is.”

The shadow cabinet secretary, Chris Matheson, also criticised the move, saying it would be a “spectacular act of desperation for Theresa May to be giving away knighthoods in a bid to win votes for her botched Brexit deal”.

He added: “This stinks of cronyism from the prime minister. We need to know if anything has been promised in exchange for this honour.”

Chris Green, a Conservative MP who followed the former Brexit and foreign secretaries, David Davis and Boris Johnson, in resigning from his Department for Transport job over May’s Chequers plan in July, told the Financial Times: “They will use whatever patronage is available to them. They are feeling the heat.”

Will Dry, the co-founder of the Our Future, Our Choice anti-Brexit campaign group that focuses on younger people, highlighted the strength of Tory opposition to the prime minister’s Brexit deal. He said: “If this is May’s plan to win them round, will a knighthood mean anything by the time this Brexit mess is all over?”

Hayes, 60, the MP for the Lincolnshire constituency South Holland and the Deepings, has been vocal about his discomfort with the deal.

A week ago, he told his local paper, the Spalding Guardian: “Whilst I can live with much of this agreement, I have been clear that we must look again at the transition arrangements and how they end.”

He added that he hoped May would listen to Tory opposition and come up with a deal that he found more acceptable.

May was critical of the former prime minister, David Cameron, for handing out numerous honours as he left office in 2016. She has been quoted as saying she “retched” when she saw the list.

Responding to the allegations against May on Friday, a Downing Street spokesman highlighted that Hayes had said he remained undecided on the deal.

Hayes had not responded to the Guardian’s request for comment but was quoted by the Sun as saying: “I still need a lot of persuading to vote for this [May’s Brexit deal].”