The Conservative backbench MP John Hayes has been granted a knighthood.
Number 10 said the Queen had approved the honour for the South Holland and The Deepings MP, who was a transport minister until a reshuffle in January.
Sir John, a long-standing sceptic about the UK’s membership of the EU, has been critical of the PM’s Brexit deal.
Labour shadow minister Chris Matheson said it would be an “act of desperation” for the PM “to be giving away knighthoods” to win deal backing.
The prime minister is due to travel to Brussels on Saturday ahead of the EU summit on Sunday, when the Brexit Withdrawal Agreement is due to be signed off.
Sir John – a Eurosceptic – has told the Sun that the honour has no bearing on how he will vote on the deal.
Speaking to his local newspaper, Spalding Today, Sir John said he was “immensely proud” to receive the knighthood.
“I am very proud to be MP for the area in which I live – I see it as something for everyone in the area, not just for me,” he said.
The awarding of a knighthood to an MP outside of the usual New Year’s and Queen’s birthday honours lists is not unprecedented.
Conservative ex-defence minister Mike Penning and ex-whip Robert Syms lost their roles in the post-election reshuffle in 2017 and were given knighthoods months later.
Former cabinet minister Eric Pickles, who was a casualty of David Cameron’s reshuffle following the Tories’ 2015 general election victory, received his knighthood later that year.
Sir John served in a series of ministerial roles from 2010 to January 2018 and was Home Office security minister from May 2015 to June 2016, while Mrs May was home secretary.
The Queen has also approved the appointment of Tory deputy chief whip Christopher Pincher and Labour whip Mark Tami to the Privy Council, Downing Street said.
On Twitter, Labour MP Chris Bryant congratulated Mr Tami and Sir John, paying tribute to the latter’s campaigning on acquired brain injury.
Mr Bryant and Sir John – who himself is a survivor of brain injury – jointly called a Commons debate on the condition earlier this year.