BANGKOK, May 20 (Reuters) - Thailand is seeking to increase its 2024 fiscal budget by 122 billion baht ($3.4 billion) to fund its signature handout scheme, its Budget Bureau chief said on Monday, only a month after a delayed 3.48 trillion baht budget was passed.

The announcement comes as the government forges ahead with plans to release fiscal stimulus with a handout of 500 billion baht ($13.93 billion) by the fourth quarter this year, the flagship policy of the ruling Pheu Thai party.

The increased budget would need cabinet approval, budget bureau director Chalermphol Pensoot told Reuters, and would bring the new budget to 3.6 trillion baht.

Thailand has been rushing to find sources of funding for the scheme, having already agreed to widen the 2025 budget deficit by 152.7 billion baht ($4.17 billion), bringing next year's budget to 3.752 trillion baht.

Economists and two former central bank governors have criticised the programme as fiscally irresponsible. The government rejects that and says it is necessary to jumpstart an economy in crisis.

The state-planning agency on Monday trimmed its 2024 outlook to 2.0% and 3.0%, from a previous forecast of 2.2% to 3.2%, despite growth in the first quarter beating expectations at 1.5%.

Last month, the finance ministry cut 2024 economic growth at 2.4%, down from 2.8%, but said it could reach 3.3% if the stimulus plan is deployed in the fourth quarter as planned.

The so called "digital wallet" programme entails giving 10,000 baht each to 50 million people to spend in their localities within six months. The central bank had recommended it target vulnerable groups only.

The government had previously said the scheme would be financed by the 2024 and 2025 fiscal budgets and funding from the state-owned Bank for Agriculture and Agricultural Cooperatives. ($1 = 35.8900 baht) (Reporting by Satawasin Staporncharnchai; Writing by Orathai Sriring and Chayut Setboonsarng; Editing by Martin Petty)