MPs have expressed concern about human rights and democracy in Turkey.
The Commons Foreign Affairs Committee has said the country is at a crossroads and must now choose between recovery or repression.
The committee said the Foreign and Commonwealth Office (FCO) seemed to be unable to provide independent analysis of last July's coup attempt, and so took the Turkish government's account of events after Ankara blamed it on a plot by the Gulenist movement.
MPs warn that in pushing wider British interests, the FCO could be perceived as not giving enough priority to UK values on human rights in Turkey.
Committee chairman Crispin Blunt said: "Turkey is an important strategic partner facing a volatile period.
"It needs and deserves our support, but that support needs to include our critique where Turkish policy is not in its own, or our, joint long-term interests: these are regional security and stability as well as strong and accountable institutions in Turkey.
"The current purges by the Turkish government amount to a root-and-branch attempt to eradicate the Gulenist movement from positions of public influence, but they have also extended beyond that to affect opposition and pro-Kurdish activists.
"Large numbers have been punished on the basis of a broad and vague definition of 'terrorism' and a worryingly low threshold of evidence. Many of those dismissed and detained have been punished without trial or access to the evidence against them. There are alarmingly inadequate avenues for redress.
"These purges risk undermining Turkey's reputation, its economy, the UK's ability to trade there, and the capabilities of the Turkish military against shared enemies such as Isil (another term for Islamic State).
"More fundamentally, they undermine the values of human rights and democracy in Turkey, already significantly weakened before the coup.
"We met President (Recep Tayyip) Erdogan during our visit, and he has made himself as central to 21st century Turkey as Mustafa Kemal Ataturk was in the 20th century.
"But now is a profound moment of decision for him and his divided country. Whether he secures an executive presidency or not, the choices that President Erdogan now makes will determine whether Turkey will be a repressive or a recovering state.
"Avoiding catastrophe, and instead shaping a positive outcome, is clearly in the interest of the UK's economy, security, and values.
"The FCO must help Turkey reinforce accountable state institutions, while also developing ties far beyond them: the UK needs a deeper and therefore more durable relationship with the Turkish people, whichever background they hold, while working to uphold the values of human rights and democracy that benefit them all."
Shadow foreign secretary Emily Thornberry said Turkey is a "vital ally, partner and friend" to the UK and so it is "essential" for the Foreign Office to properly understand the country.
The Labour frontbencher went on: "So I'm particularly concerned by the corrosive impact of cuts to the FCO's budget on its ability to fully understand and analyse developments in Turkey – resulting in a situation where ministers readily admitted that last year's attempted coup in Turkey caught them completely by surprise.
"Equally concerning is the decision, driven by the Treasury, to downgrade the importance of human rights in the FCO's work. The committee's report is a timely reminder that we need to find a proper balance between values and interests in foreign policy, and that in Turkey and elsewhere, this Government is increasingly getting that balance wrong."
An FCO spokesman said: "We welcome this report on the UK's relations with Turkey and its praise for the UK's approach.
"As the report acknowledges, Turkey remains a vital strategic partner for the UK. It sits on the front line of some of the most difficult and serious challenges we face.
"The UK works closely with Turkey on regional stability and security, partnering on counter-terrorism, and improving markets for British exporters and growing inward investment, and this will remain unchanged following our EU exit.
"We recognise the challenges that Turkey faces and we condemned last July's coup attempt, which was a shocking attack on Turkish democracy.
"We have always said that the Turkish government's response to the coup attempt must be measured. The UK consistently raises human rights issues with our Turkish counterparts.
"We note the report's recommendations and will consider these and respond in due course."