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Merino opened the scoring with a header in the 35th minute and doubled Spain's lead in the 56th with another header.

Bulgaria's woes deepened in the 79th minute when defender Atanas Chernev turned a cross into his own net while Mikel Oyarzabal completed the rout in added time, converting a penalty after Merino was brought down in the area.

Spain's dominance in Group E remains unchallenged, with Luis de la Fuente's side leading the standings with 12 points from four matches, scoring 15 goals and conceding none.

It is followed by Turkey on nine points, while Georgia is third on three points. Bulgaria remains bottom of the group after suffering its fourth consecutive defeat.

Spain will travel to Georgia in November before hosting Turkey in Seville in its final qualifier with a potentially massive goal-difference advantage.

"We are edging closer (to securing a World Cup spot), but until it's 100% certain, nothing is final," Merino said. "All the teams are very well prepared tactically nowadays and the number of chances our team creates is commendable.

"This is the way forward. On the day we're on target, we'll score lots of goals. When we're not, we'll still have the game under control."

Despite missing several key starters like Lamine Yamal, Rodri and Nico Williams, the European champion dominated the match in Valladolid, enjoying nearly 80 percent possession and creating 33 chances against a Bulgaria side that offered little resistance.

Despite the lopsided scoreline, Spain could have won by an even larger margin but for wasteful finishing and a string of outstanding saves from goalkeeper Svetoslav Vutsov.

Spain came close to breaking the deadlock early on, with Pedri hitting the crossbar in the 19th minute and Samu Aghehowa twice denied by Vutsov from close range.

The Bulgarian keeper was also called into action to make a superb save to tip Merino's strike over the bar in the 34th minute.

The breakthrough finally came a minute later when Pedri delivered a pinpoint cross from the left to Robin Le Normand at the far post. The defender cushioned a header back into the centre for Merino, who nodded home to the goalkeeper's left.

After the break, Spain brought on Borja Iglesias to replace Samu, who had missed several first-half chances.

Iglesias nearly scored twice early in the second half, but it was Merino who doubled Spain's lead in the 56th minute. Alex Grimaldo delivered a perfectly placed cross from deep on the left, and Merino met it with a stooping header at the far post, sending the ball into the net to the goalkeeper's right.

Spain's third goal came in the 79th minute when Chernev was trying to prevent Aleix Garcia's cross reaching Iglesias inside the six-yard box, with Oyarzabal wrapping up the easy win from the spot.