David Cameron is being urged by EU Out campaigners to block moves to admit Turkey to the 28-nation bloc.
Conservative former shadow home secretary David Davis said Turkey was a "state in crisis" and the consequences of allowing it to join the EU could be "catastrophic".
His intervention comes as Mr Cameron prepares to travel to Brussels on Thursday for further talks on a deal that would see Turkey help stem the flow of migrants into Europe in return for a speeding up of the accession process and visa-free travel for Turkish nationals in the Schengen area.
But in a letter to Mr Cameron, Mr Davis - a spokesman for the Grassroots Out campaign - warned that democracy, human rights and the rule of law had all diminished since Recep Tayyip Erdogan had moved from prime minster to become president.
"Erodgan's administration is moving in a dangerous direction, embracing political Islam and moving in a direction inconsistent with the values of a Western democracy," he wrote.
"The relationship between some parts of the Turkish government and jihadists in Syria and the porous border between the two countries means it is possible visa-free access to Schengen will become open access for violent extremists to all the continent of Europe."
He added: "Britain must not walk into this blindly. The consequences could be catastrophic and may prove irreversible."