About three months after Mr Deb Nath Dipangar, 34, arrived in Singapore from Bangladesh to begin a job as a construction worker, he found a better offer - potentially paying more than the $468 a month he was receiving then.

His wages had been lower than initially promised and he decided to transfer to another employer here. However, he said he did not get an in-principle approval (IPA) letter, spelling out his new terms of employment, when he did so.

Similar situations faced by foreign workers who change employers add to issues - such as incomplete documents - that they may encounter involving IPA letters.

About 15 per cent of non-Malaysian work permit holders in a recent survey said they did not receive the letters before coming here, up from 4 per cent in 2014.

The study last year, commissioned by the Ministry of Manpower (MOM), polled about 2,500 work permit holders and 500 S Pass holders randomly.

Mr Alex Au, vice-president of non-profit group Transient Workers Count Too (TWC2), said workers outside Singapore need IPAs to pass through Changi Airport. Workers who are already here need not go through immigration again.

So, even though the MOM requires employers to provide workers with a copy of the IPA, some may not do so, Mr Au added.

There also seems to be more workers changing firms, he said.

From 2005 to October 2017, about 128,000 work permit holders changed employers before the end of their permit terms.

Another 400 did so after completing their terms, then Manpower Minister Lim Swee Say told Parliament in November 2017.

Among the workers who turn to TWC2 for help, Mr Au estimates that 80 per cent to 90 per cent of those who have transferred firms say they have no IPA letters.

This could be a problem in the event of a dispute, as the letter contains the terms and conditions of employment, including the basic salary. It serves as a de facto contract in the absence of other written agreements. Employers who fail to send out IPA letters before workers leave for Singapore may be fined up to $10,000.

Another challenge that foreign workers face here is receiving incomplete IPA letters, said Ms Desiree Leong, legal consultant at the Humanitarian Organisation for Migration Economics (Home).

While it is mandatory for employers and agencies to send the full letter to foreign workers before they depart for Singapore, it is not required to clear immigration, she said. "The front or cover page, which contains no information on salary, suffices."

Agents and employers may deliberately give workers only that part of the IPA, keeping them uncertain of their legal rights and raising their vulnerability to exploitation.

Noting that some workers have been deceived about their salary, Ms Leong said about a fifth of those who approach Home for help with their pay have incomplete IPA letters.

As workers may receive only some pages of the IPA - which can run to more than eight pages - for their use to enter Singapore, they may not realise it is the IPA, said employment lawyer Wiyatno Gerald Mursjid.

The MOM has been taking steps to ensure workers receive their IPA letters before coming here, making it a requirement since October last year for them to take the letters to their Settling-in Programme.

From February to April, it issued advisory letters to 60 employers whose workers either did not have their IPA letters or had an incomplete set during the SIP.

Last month, five employers and an employment agency were investigated for failing to provide the full IPA letter before a worker's departure. Investigations found that the employers had fulfilled their obligations. Enforcement action against the agency, which is not linked to the employers, is pending.

Between February and April, 97 per cent of workers attending the SIP had their full IPA letters, said the MOM. Those who did not have the complete set were given a copy by the Migrant Workers' Centre.

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