WARSAW, Jan 28 (Reuters) - Poland should not help wealthy borrowers who took out Swiss franc-denominated mortgages to buy homes, Deputy Prime Minister Mateusz Morawiecki said.

Swiss franc loans have become a political headache for Poland’s ruling Law and Justice (PiS) party. President Andrzej Duda, who was elected last year with PiS backing, promised their conversion into zlotys during his campaign, but the costs of such an operation could badly hurt Polish banks.

Fitch rating agency has warned it could cut Poland’s rating if the country’s decision to convert the Swiss franc-denominated mortgages significantly undermines the health of the banking sector.

This month Duda set out a plan to saddle lenders with the costs of conversion - potentially billions of euros - and sent a draft bill to the financial supervisor KNF to estimate the cost to banks.

“I think that on the basis of this bill we should not help people who are richer or generally rich people, or these who have bought several flats, or even one flat, but can afford it,” Morawiecki told private broadcaster TVN24 late on Wednesday.

The bill submitted by the president’s office contains no income or wealth thresholds.

“Banks have already created a multi-million fund and I think this is a direction we should consider, increase the fund and help those who struggle,” said Morawiecki, a former bank executive.

Seeking to balance his comments, Morawiecki added it would be prudent to wait up to four more months for an estimate of the costs to the banking sector of the president’s bill.

Morawiecki also said owners of Polish lenders, which include large western European banks such as Unicredit and Santander, may sue Poland in international arbitration courts over a conversion.

More than 500,000 Poles took out Swiss franc mortgages, mostly in 2007 and 2008, to benefit from lower Swiss interest rates. But the franc has risen by 80 percent against the zloty since then, leaving many borrowers with loans worth more than the flats they were used to purchase.

Polish Finance Minister Pawel Szalamacha said on Thursday he would call for further work on the president’s bill.

“This is a real problem that concerns several hundred thousand borrowers ... The banking sector has done nothing or close to zero to solve this problem,” Szalamacha said.

The Polish banking sector holds Swiss franc loan portfolios worth about 8 percent of Poland’s national output. ($1 = 4.1181 zlotys) (Reporting by Wiktor Szary and Marcin Goclowski; Writing by Marcin Goettig; Editing by Gareth Jones)