Spanish MP Francisco Martinez Vazquez, former Security Minister, said the Catalan government was acting “in a very improper way” and the Spanish government would step in if there was an independence declaration.
But, in a live BBC interview, presenter John Humphrys cut in to say such action could lead to “civil war”.
Mr Vazquez replied he hoped Catalan chiefs would “step back” and reconsider its options.
Speaking on the Today programme, on BBC Radio 4, Mr Vazquez said Spain could not accept a “breach of the law”.
He said: “Well if they do go in a unilateral way to an independence declaration, just supported by a part of Catalan society and a part of the regional department, of course the Government will do what the law and the constitution provides the Government to do.
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“It is not only the Government, it is also all the other institutions that make us be a country under the rule of law which is the constitutional court, which is a different court that is acting.
“The prosecutors and everyone reacting to protect the national state.”
Humphrys said a tough stance was a “risk”.
He said: “But you risk, of course, serious disturbance then, there’s been talk, as you well know, of civil war.”
But Mr Vazquez, of the liberal-conservative People’s Party, replied Spain would not accept a “breach of the law”.
He said: “No, I’m sure that won’t happen. I really hope that the regional government, which is acting in a very improper way, will step back from what they’re doing.
“But we cannot accept that just because there is someone saying there’s going to be a disturbance, then we have to just accept the breach of the law.”
Catalonia is set to ignore threats from Brussels and Madrid and officially declare independence from Spain.
The region’s leader has now confirmed independence will be declared – whether Spain or the EU like it or not.
Carles Puigdemont said the region’s rules stated a declaration of independence was the legal next step after the ‘yes’ victory last week.
He said: “The declaration of independence, that we don’t call a ‘unilateral’ declaration of independence, is foreseen in the referendum law as an application of the results.
“We will apply what the law says.”