Kirsty Williams has been accused of a "pick and mix" approach to government after voting against her colleagues on Brexit.
The Liberal Democrat AM backed an attempt by Plaid Cymru in the Welsh Assembly to oppose the triggering of Article 50 - the process by which Britain will leave the European Union.
The Labour Party had voted against Plaid’s move, pitting Ms Williams against her cabinet colleagues. The Brecon and Radnorshire AM cut a deal with Labour leader Carwyn Jones to join his government after last May’s elections.
Assembly members debated Brexit on Tuesday when Plaid put forward a call for AMs to oppose the start of the Brexit process unless assurances are given over Wales’ participation in the single market.
The Senedd had debated a joint Labour-Plaid Cymru Brexit white paper that called for continued participation in the single market.
Ms Williams was one of just 10 AMs to back Plaid’s amendment. A majority of AMs - 38 for and 18 against - backed a Plaid-Labour motion that noted the UK government intend to trigger Article 50 but did not oppose it.
The motion called for consultation on the negotiations on any final deal.
The votes were only advisory and will not affect the Article 50 process.
Ms Williams’ vote has however led to Welsh Conservative leader Andrew RT Davies questioning if the Welsh Government is still bound by collective responsibility - the convention that cabinet members publicly support all governmental decisions taken in cabinet even if they do not privately agree with them.
Mr Davies said: "Carwyn Jones needs to clarify whether his ministers are still bound by collective responsibility, because at present it appears that the cabinet secretary for education can opt out whenever she pleases.
"It makes the First Minister look incredibly weak.
"It cannot be government by ’pick and mix’. It’s a longstanding tradition that ministers adhere to the principle of collective responsibility, and where an individual feels that they can no longer support the government’s position they remove themselves from the cabinet.
"Whilst I personally respect Kirsty Williams’ right to take this position, it is on the basis of collective responsibility that she joined the government and Labour backbenchers would have every right to be furious if they feel that she is receiving special treatment."
A spokesman for Ms Williams said she "fully supports" the Welsh Government’s Brexit white paper but had support Plaid’s amendment: "The Liberal Democrats strongly believe that the UK government must commit to securing Britain’s membership of the single market ahead of triggering Article 50. Therefore Kirsty voted for the amendment."