The email, obtained by the Sunday Herald despite an attempt to censor its contents, shows the Scottish Health Secretary was vehemently against removing acute mental health services from Monklands Hospital because of the impact on his Airdrie & Shotts seat.
Neil wrote that the move to remove services was "prejudiced" and said: "I don't agree with your recommendations in relation to not locating one of the new units in Monklands."
He later ordered NHS Lanarkshire to rewrite its plans and retain beds at Monklands, despite official warnings it would mean a "less than optimal service" at the hospital, where asbestos is "ubiquitous".
Despite this, Neil told the Scottish Parliament he had delegated decisions on Monklands to a deputy minister as "it lies in my constituency". The ministerial code says ministers must avoid conflicts of interests when taking decisions affecting their constituencies.
Labour claim Neil "deceived" the Parliament and have urged Alex Salmond to sack him. The First Minister, who previously cleared Neil of breaching the ministerial code over the affair, last week refused to dismiss him. "To define it purely as a constituency issue ignores the fact that the health service affects and serves all of the population," he said.
But an email written by Neil to NHS Lanarkshire on August 9, 2012 suggests that constituency issues were vital. The then Health Secretary Nicola Sturgeon had just agreed to concentrate mental health beds at Wishaw and Hairmyres, and remove them from Monklands.
With NHS Lanarkshire due to finalise the plan on August 23, Neil was asked if he would endorse the board's decision in a press release.
NHS Lanarkshire blacked out Neil's reply, but the Sunday Herald has seen the contents. After asking the board to delay a decision, Neil declined to contribute.
"While I support the overall strategic approach, I don't agree with your recommendations in relation to not locating one of the new units in Monklands, which covers a much more deprived catchment area than East Kilbride," he said. "The proposals are not consistent with an anti-poverty strategy. My reading of the analysis of why Monklands shouldn't retain a mental health unit is that it is essentially prejudiced on the economics of the PFIs at both Wishaw and Hairmyres.
"Given the level of poverty and deprivation in Airdrie this will just not be possible if people on low incomes have to pay to get to Wishaw or Hairmyres. I strongly suggest that this release includes the details of planned investment at Monklands Hospital, otherwise local people will be even further incensed yet again by the actions of NHS Lanarkshire."
To allow Neil to stay involved, the board delayed its decision until September 26. But on September 5, a reshuffle saw Neil promoted to Health Secretary. Within days he undertook a "review" of the mental health plans, and on September 26 his private secretary told fellow Government officials Neil's "clear view" was to keep beds at Monklands. NHS Lanarkshire was asked to "reconfigure its plans accordingly".
Later that day, Neil stepped aside due to "a perception of a conflict of interest". Labour argue Neil had already taken the decision.
Labour health spokesman Neil Findlay said: "It makes clear that Alex Neil was going to oppose the decision to close mental health beds at Monklands, because of the impact it would have on his constituency.
"Alex Neil said he wouldn't handle the decision because it was a constituency matter. Alex Salmond this week said it wasn't a constituency matter. The SNP's defence is all over the place as they scrabble to defend a minister interfering in NHS decisions. Alex Neil's position is untenable. He must resign."
A Scottish Government spokeswoman said: "On his appointment as Health Secretary, Mr Neil wished to review a number of key decisions, including the proposals about to be put forward by the board of NHS Lanarkshire on mental health services. With over 500,000 people resident in NHS Lanarkshire's area, Mr Neil addressed his concerns on the service change to the region as a whole. He was clear in his view that acute mental health facilities would be best retained at Wishaw General Hospital, Monklands Hospital, and with a unit at Hairmyres Hospital."
Regarding asbestos at Monklands, the Government said: "Asbestos is only dangerous when disturbed. NHS boards have a responsibility to ensure asbestos is safely managed ... It should not be removed unnecessarily."