Rajoy's latest escalation of a war of words with Britain and Gibraltar centers round Gibraltar's decision to build an artificial reef to help its fishing catch. Spain argues the reef will block its fishing vessels but critics say Spain already has built similar reefs along its coast by dropping concrete blocks into the sea.
Rajoy's parliamentary opponents are reported lobbying to garner enough support for a vote of no-confidence against the prime minister on charges he accepted illegal cash from a ruling party slush funds. Earlier this month Rajoy defused a parliamentary censure campaign by admitting he made an error accepting cash from a party official he trusted at that time.
The scandal has already gone to court and a coming trial of a party official is likely to revive the charges.
Britain took over Gibraltar following a treaty 300 years ago and disputes on the British presence is endemic.
"But many people suspect that Gibraltar is not even the real issue at stake here," Deutsche Welle German radio said in a dispatch.
"They say the Spanish government is using the controversy as a distraction from a more pressing problem. The governing party of Mariano Rajoy has been facing allegations of widespread corruption and on the first of August the prime minister appeared before congress to face questions about the affair. Gibraltar, the theory goes, is just a smokescreen."
Rajoy's opposition critics are more forthright and argue Gibraltar and Rajoy's political troubles are linked.
The prime minister met with King Juan Carlos Friday and announced his government would take more "legal measures" to protect Spanish interests.
Spanish police already have toughened border checks, delaying two-way road traffic by several hours. New moves being considered by Madrid include a 50-euro ($67) levy on every vehicle entering or leaving Gibraltar. The border levy, if imposed on Spanish workers in Gibraltar, could add hundreds more to Spain's burgeoning ranks of the jobless, but Madrid hasn't said if cross-border workers will be exempt.
Rajoy's government is fighting to recover approval ratings after failing to meet economic recovery and employment targets. The corruption scandal has hurt its credibility, media opinion polls indicated.
In comments to El Pais, Rajoy called for talks among Britain, Spain, Gibraltar and the neighboring regional government of Andalusia, whose Cadiz province shares a three-quarter-mile land frontier with Gibraltar.
Spain, Rajoy said, "will take proportionate measures that don't discriminate against anyone, but of course it will take legal measures to defend the interests of Spanish citizens.
"But I hope this goes no further," Rajoy said.
British Prime Minister David Cameron has accused Rajoy of escalating the diplomatic row. Rajoy's opposition critics say they won't be distracted from the campaign over corruption charges against the government.