Budget carriers Frontier Group Holdings and Spirit Airlines Inc on Monday unveiled plans to create the fifth-largest U.S. airline in a $2.9 billion tie-up likely to tighten competition against traditional carriers.

The proposal to form a new no-frills carrier controlled by Frontier Airlines pushed up shares of Spirit as much as 18.7%, though several analysts pressed the airlines over possible difficulties in obtaining regulatory approval.

The merger, which is expected to close in the second half of 2022, is projected to result in synergies of $500 million a year, mainly through operational savings.

Read more: Reuters