Critics have accused the Prime Minister of being "too weak" to confront the hard Brexiteers in her own party.
As the BBC's Laura Kuenssberg noted, the number of signatories on the letter is significantly greater than the number of MP's needed for a leadership challenge: 48. She was left weakened by last year's disastrous election.
"The withdrawal agreement has to take into account the future relationship", Brexit Secretary David Davis said on Tuesday, when asked about Bloomberg's report that the money could be linked to the European Union delivering its promises on trade.
Her government will publish a written statement Wednesday setting out its response to the European Union's negotiating guidelines on the transition period.
On Tuesday, Davis outlined his idea for future collaboration, promising the other 27 member countries that the United Kingdom won't try to undercut them by tearing up regulations when it leaves.
The largely backbench politicians, including former Tory leader Iain Duncan Smith and vocal Brexiteer Jacob Rees-Mogg, said they wanted to reassure Mrs May of their "continued, strong backing for the clear vision of an internationally-engaged, free-trading, global Britain" and that they had some "suggestions about how it could be achieved".
The lawmakers' letter was dated February 16.
After months of internal divisions, May is aiming to win support on Thursday from her inner circle of cabinet ministers for her blueprint for the post-Brexit trade deal between the United Kingdom and the EU. "Any implementation period must not restrain the United Kingdom from negotiating or signing other trade agreements".
In their letter, Tory Eurosceptics including former ministers Priti Patel and Iain Duncan Smith warned the United Kingdom must have full control over laws after Brexit and must not become a "rule taker".
"The UK must be free to start its own trade negotiations immediately", the letter says.
The EU has said the transition phase should end on 31 December, 2020, the end of its budget period.
That the United Kingdom is free to start negotiating new trade deals with non-EU countries immediately after Brexit in March 2019, and that any "implementation period" doesn't restrict the UK's ability to do so.
Responding, Labour MP Chris Leslie said on behalf of the pro-EU group Open Britain: "The ERG have long felt they can dictate Government policy and they are brazenly advocating the hardest of hard Brexits".
While the letter stopped short of telling May to drop her transition plan altogether, what it called for is quite different from the "no change" period that May and her ministers have told businesses they can expect next year.