Japanese drugmaker Takeda has announced that it is considering buying Ireland-domiciled drugmaker Shire, although it stressed that the offer was “at a preliminary and exploratory stage” and it had not approached Shire’s board.

Market speculation and subsequent pressure from the UK Takeover Panel had prompted Takeda to officially confirm its interest in Shire, although the company added that any offer would have to align with its “disciplined approach” to acquisitions. The Takeover Panel has given Takeda until Apr. 25 to make a firm bid.

Shire has been the subject of much takeover speculation in recent years and there are expectations that Takeda’s interest could spark a bidding war, or at least a counter bid.

Late last year, Pfizer was rumored to have hired advisers to work on a potential offer and, in October 2014, AbbVie called off its proposed £32 billion takeover of Shire following new US rules on inverted mergers.

Investors have been largely skeptical about Takeda’s potential move, with many questioning the Osaka-based company’s ability to finance the deal. The Japanese company, currently valued around $39 billion, is smaller than rare disease specialist Shire, which is worth around $35 billion.

 Mizuho Securities analyst Hiroshi Tanaka said in a note to clients quoted by Reuters news agency: “The impression left by the news is that the acquisition would be an overreach.”  Analysts also said that any deal was expected to involve some form of equity, whether a share swap or share issue, which could be highly dilutive for Takeda shareholders.

Ratings agency Moody’s also warned that Takeda would add more debt to an already weakened balance sheet, arguing that the size of the deal and its global complexity could make a merger difficult to execute.

Takeda added $3.5 billion of debt last year from the takeover of US oncology company Ariad Pharmaceuticals. That deal, costing $5.2 billion, was Takeda’s largest since it paid $13.7 billion for Swiss drugs company Nycomed in 2011.

An acquisition of Shire, said Takeda, could create a global biopharmaceutical leader, boosting its presence in the US market as well as in the areas of gastroenterology, oncology and neuroscience.

Several major pharma deals have been announced so far this year. These included in January Sanofi’s purchases of Bioverativ and Ablynx, Roche’s takeover of US-based Ignyta and Celgene’s agreement to acquire Juno Therapeutics. In February, US drugmaker Merck & Co. agreed to buy Australia’s Viralytic, while late last month GlaxoSmithKline said it would pay $13 billion to buy Novartis’ share in its consumer healthcare joint venture.